Light-emitting element, light-emitting device, electronic device, lighting device, and organic compound

ABSTRACT

To provide a light-emitting element with an improved reliability, a light-emitting element with a high current efficiency (or a high quantum efficiency), and a novel dibenzo[f,h]quinoxaline derivative that is favorably used in a light-emitting element which is one embodiment of the present invention. A light-emitting element includes an EL layer between an anode and a cathode. The EL layer includes a light-emitting layer; the light-emitting layer contains a first organic compound having an electron-transport property and a hole-transport property, a second organic compound having a hole-transport property, and a light-emitting substance; the combination of the first organic compound and the second organic compound forms an exciplex; the HOMO level of the first organic compound is lower than the HOMO level of the second organic compound; and a difference between the HOMO level of the first organic compound and the HOMO level of the second organic compound is less than or equal to 0.4 eV.

This application is a continuation of copending U.S. application Ser.No. 14/807,426, filed on Jul. 23, 2015 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention also relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a light-emitting element, alight-emitting device, an electronic device, a lighting device, adriving method thereof, or a manufacturing method thereof. Oneembodiment of the present invention further relates to an organiccompound which can be used in a light-emitting element, a light-emittingdevice, an electronic device, and a lighting device.

2. Description of the Related Art

A light-emitting element using an organic compound as a luminous body,which has features such as thinness, lightness, high-speed response, andDC drive at low voltage, is expected to be used in a next-generationflat panel display. In particular, a display device in whichlight-emitting elements are arranged in a matrix is considered to haveadvantages in a wide viewing angle and excellent visibility over aconventional liquid crystal display device.

The light emission mechanism of a light-emitting element is said to beas follows: when a voltage is applied between a pair of electrodes withan EL layer including a luminous body provided therebetween, electronsinjected from a cathode and holes injected from an anode recombine inthe light emission center of the EL layer to form molecular excitons,and energy is released and light is emitted when the molecular excitonsreturn to the ground state. Singlet excitation and triplet excitationare known as excited states, and it is thought that light emission canbe achieved through either of the excited states.

In order to improve element characteristics of such a light-emittingelement, improvement of an element structure, development of a material,and the like have been actively carried out (see, for example, PatentDocument 1).

REFERENCE

[Patent Document]

[Patent Document 1] Japanese Published Patent Application No.2010-182699

SUMMARY OF THE INVENTION

When light-emitting elements are developed, one of important points forcommercialization is to improve the reliability of the elements. Inorder to improve the reliability of the elements, an element structureis needed, which can control the carrier balance and improve therecombination probability of carriers in an EL layer of a light-emittingelement. Therefore, it is an object to provide a highly reliablelight-emitting element in the following way: an EL layer is made to havea desired structure so that carriers are transferred efficiently in alight-emitting layer. A high current efficiency (or a high quantumefficiency) is also important to decrease the amount of current neededfor driving the element and to improve the reliability.

In one embodiment of the present invention, a light-emitting elementwith an improved reliability is provided. In addition, a light-emittingelement with a high current efficiency (or a high quantum efficiency) isprovided. Furthermore, a novel organic compound that is favorably usedin a light-emitting element which is one embodiment of the presentinvention is provided. In another embodiment of the present invention, alight-emitting element, a light-emitting device, an electronic device,or a lighting device with a high emission efficiency and a highreliability, in which the above organic compound is used as an ELmaterial, is provided. In another embodiment of the present invention, anovel material is provided. In another embodiment of the presentinvention, a novel light-emitting element and a novel light-emittingdevice are provided. Note that the descriptions of these objects do notdisturb the existence of other objects. In one embodiment of the presentinvention, there is no need to achieve all the objects. Other objectswill be apparent from and can be derived from the descriptions of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a light-emitting elementincluding an EL layer between an anode and a cathode. The EL layerincludes a light-emitting layer; the light-emitting layer contains afirst organic compound having an electron-transport property and ahole-transport property, a second organic compound having ahole-transport property, and a light-emitting substance; the combinationof the first organic compound and the second organic compound forms anexciplex; the HOMO level of the first organic compound is lower than theHOMO level of the second organic compound; and a difference between theHOMO level of the first organic compound and the HOMO level of thesecond organic compound is less than or equal to 0.4 eV.

Another embodiment of the present invention is a light-emitting elementincluding an EL layer between an anode and a cathode. The EL layerincludes a light-emitting layer; the light-emitting layer contains afirst organic compound having an electron-transport property and ahole-transport property, a second organic compound having ahole-transport property, and a light-emitting substance; the combinationof the first organic compound and the second organic compound forms anexciplex; the first organic compound includes a 6-memberednitrogen-containing heteroaromatic ring and a carbazole skeleton anddoes not include a triarylamine skeleton; and the second organiccompound includes a triarylamine skeleton.

Another embodiment of the present invention is a light-emitting elementincluding an EL layer between an anode and a cathode. The EL layerincludes a light-emitting layer; the light-emitting layer contains afirst organic compound having an electron-transport property and ahole-transport property, a second organic compound having ahole-transport property, and a light-emitting substance; the combinationof the first organic compound and the second organic compound forms anexciplex; the first organic compound includes a 6-memberednitrogen-containing heteroaromatic ring and a bicarbazole skeleton anddoes not include a triarylamine skeleton; and the second organiccompound includes a triarylamine skeleton.

In the above structure, the bicarbazole skeleton is a 3,3′-bicarbazoleskeleton or a 2,3′-bicarbazole skeleton.

In each of the above structures, the light-emitting substance is aphosphorescent compound.

In each of the above structures, the EL layer includes a hole-transportlayer; the hole-transport layer is in contact with the light-emittinglayer and contains a third organic compound having a hole-transportproperty; and the HOMO level of the third organic compound is lower thanthe HOMO level of the second organic compound.

In each of the above structures, the first organic compound isrepresented by the following general formula (G0).

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group.

Another embodiment of the present invention is an organic compoundrepresented by the following general formula (G0).

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group.

Another embodiment of the present invention is an organic compoundrepresented by the following general formula (G1).

In the formula, R¹ to R²⁴ represent independently hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 5 to 7 carbonatoms, or a substituted or unsubstituted aryl group having 6 to 13carbon atoms; and Ar represents a substituted or unsubstituted arylenegroup having 6 to 25 carbon atoms or a single bond. Preferably, anarylene group in Ar does not include an anthracenylene group.

Another embodiment of the present invention is an organic compoundrepresented by the following general formula (G2).

In the formula, R¹ to R²⁴ represent independently hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 5 to 7 carbonatoms, or a substituted or unsubstituted aryl group having 6 to 13carbon atoms; and Ar represents a substituted or unsubstituted arylenegroup having 6 to 25 carbon atoms or a single bond. Preferably, anarylene group in Ar does not include an anthracenylene group.

Another embodiment of the present invention is an organic compoundrepresented by the following general formula (G3).

In the formula, R¹ to R²⁴ represent independently hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 5 to 7 carbonatoms, or a substituted or unsubstituted aryl group having 6 to 13carbon atoms; and Ar represents a substituted or unsubstituted arylenegroup having 6 to 25 carbon atoms or a single bond. Preferably, anarylene group in Ar does not include an anthracenylene group.

Examples of the alkyl group having 1 to 6 carbon atoms in each of theabove general formula (G0), general formula (G2), and general formula(G3) include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a sec-butyl group, atert-butyl group, a pentyl group, an isopentyl group, and a hexyl group.Examples of the cycloalkyl group having 5 to 7 carbon atoms include acyclopentyl group, a cyclohexyl group, and a cycloheptyl group. Examplesof the aryl group having 6 to 13 carbon atoms include a phenyl group, atolyl group, a xylyl group, a biphenyl group, an indenyl group, anaphthyl group, and a fluorenyl group. Examples of the arylene grouphaving 6 to 25 carbon atoms in Ar include a 1,2-, 1,3-, and1,4-phenylene groups, a 2,6-, 3,5-, and 2,4-toluylene groups, a4,6-dimethylbenzene-1,3-diyl group, a 2,4,6-trimethylbenzene-1,3-diylgroup, a 2,3,5,6-tetramethylbenzene-1,4-diyl group, a 3,3′-, 3,4′-, and4,4′-biphenylene groups, a 1,1′: 3′,1″-terbenzene-3,3″-diyl group, a1,1′: 4′,1″-terbenzene-3,3″-diyl group, a 1,1′:4′,1″-terbenzene-4,4″-diyl group, a 1,1′: 3′,1″:3″,1′″-quaterbenzene-3,3′″-diyl group, a 1,1′: 3′,1″:4″,1′″-quaterbenzene-3,4′″-diyl group, a 1,1′: 4′,1″:4″,1′″-quaterbenzene-4,4′″-diyl group, a 1,4-, 1,5-, 2,6-, and2,7-naphthylene groups, a 2,7-fluorenylene group, a9,9-dimethyl-2,7-fluorenylene group, a 9,9-diphenyl-2,7-fluorenylenegroup, a 9,9-dimethyl-1,4-fluorenylene group, aspiro-9,9′-bifluorene-2,7-diyl group, a9,10-dihydro-2,7-phenanthrenylene group, a 2,7-phenanthrenylene group, a3,6-phenanthrenylene group, a 9,10-phenanthrenylene group, a2,7-triphenylenylene group, a 3,6-triphenylenylene group, a2,8-benzo[a]phenanthrenylene group, a 2,9-benzo[a]phenanthrenylenegroup, and a 5,8-benzo[c]phenanthrenylene group.

The alkyl group having 1 to 6 carbon atoms, the cycloalkyl group having5 to 7 carbon atoms, the aryl group having 6 to 13 carbon atoms, and thearylene group having 6 to 25 carbon atoms may each have a substituent.Examples of the substituent preferably include alkyl groups each having1 to 6 carbon atoms, such as a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, a sec-butylgroup, a tert-butyl group, a pentyl group, an isopentyl group, and ahexyl group; cycloalkyl groups each having 5 to 7 carbon atoms, such asa cyclopentyl group, a cyclohexyl group, and a cycloheptyl group; andaryl groups each having 6 to 13 carbon atoms, which form a ring, such asa phenyl group, a tolyl group, a xylyl group, a biphenyl group, anindenyl group, a naphthyl group, a fluorenyl group, and a9,9-dimethylfluorenyl group.

Another embodiment of the present invention is a light-emitting elementcontaining the organic compound represented by any of the above generalformulae (G0) to (G3).

Another embodiment of the present invention is a light-emitting deviceincluding the light-emitting element having any of the above structuresand a housing.

One embodiment of the present invention includes, in its category, inaddition to a light-emitting device including a light-emitting element,an electronic device including the light-emitting element or thelight-emitting device (specifically, an electronic device including thelight-emitting element or the light-emitting device and a connectionterminal or an operation key) and a lighting device including thelight-emitting element or the light-emitting device (specifically, alighting device including the light-emitting element or thelight-emitting device and a housing). A light-emitting device in thisspecification refers to an image display device or a light source (e.g.,a lighting device). A light-emitting device also includes, in itscategory, all of a module in which a light-emitting device is connectedto a connector such as a flexible printed circuit (FPC) or a tapecarrier package (TCP), a module in which a printed wiring board isprovided on the tip of a TCP, and a module in which an integratedcircuit (IC) is directly mounted on a light-emitting element by a chipon glass (COG) method.

In accordance with one embodiment of the present invention, a noveldibenzo[f,h]quinoxaline derivative can be provided. In accordance withone embodiment of the present invention, a light-emitting element, alight-emitting device, an electronic device, or a lighting device with ahigh emission efficiency and a high reliability, in which the abovedibenzo[f,h]quinoxaline derivative is used as an EL material, can beprovided. In accordance with one embodiment of the present invention, anovel material can be provided. In accordance with another embodiment ofthe present invention, a novel light-emitting element and a novellight-emitting device can be provided. Note that the descriptions ofthese effects do not disturb the existence of other effects. Oneembodiment of the present invention does not necessarily achieve all theeffects listed above. Other effects will be apparent from and can bederived from the descriptions of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a light-emitting layer of a light-emittingelement.

FIG. 2 schematically shows a light-emitting layer of a light-emittingelement.

FIGS. 3A and 3B each illustrate the structure of a light-emittingelement.

FIGS. 4A and 4B each illustrate the structure of a light-emittingelement.

FIG. 5 illustrates a light-emitting device.

FIGS. 6A and 6B illustrate a light-emitting device.

FIGS. 7A, 7B, 7C, 7D, 7D′ 1 and 7D′2 each illustrate an electronicdevice.

FIGS. 8A to 8C illustrate an electronic device.

FIG. 9 illustrates lighting devices.

FIGS. 10A and 10B are ¹H-NMR charts of a dibenzo[f,h]quinoxalinederivative represented by the structural formula (100).

FIG. 11 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (100).

FIG. 12 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (100).

FIGS. 13A and 13B are ¹H-NMR charts of a dibenzo[f,h]quinoxalinederivative represented by the structural formula (101).

FIG. 14 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (101).

FIG. 15 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (101).

FIGS. 16A and 16B are ¹H-NMR charts of a dibenzo[f,h]quinoxalinederivative represented by the structural formula (102).

FIG. 17 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (102).

FIG. 18 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (102).

FIGS. 19A and 19B are ¹H-NMR charts of a dibenzo[f,h]quinoxalinederivative represented by the structural formula (103).

FIG. 20 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (103).

FIG. 21 shows the ultraviolet-visible absorption spectrum and theemission spectrum of the dibenzo[f,h]quinoxaline derivative representedby the structural formula (103).

FIG. 22 illustrates the structure of each of a light-emitting element 1,a light-emitting element 2, and a comparative light-emitting element 3.

FIG. 23 shows current density-luminance characteristics of thelight-emitting element 1, the light-emitting element 2, and thecomparative light-emitting element 3.

FIG. 24 shows voltage-luminance characteristics of the light-emittingelement 1, the light-emitting element 2, and the comparativelight-emitting element 3.

FIG. 25 shows luminance-current efficiency characteristics of thelight-emitting element 1, the light-emitting element 2, and thecomparative light-emitting element 3.

FIG. 26 shows voltage-current characteristics of the light-emittingelement 1, the light-emitting element 2, and the comparativelight-emitting element 3.

FIG. 27 shows the emission spectra of the light-emitting element 1, thelight-emitting element 2, and the comparative light-emitting element 3.

FIGS. 28A and 28B show the reliability of each of the light-emittingelement 1, the light-emitting element 2, and the comparativelight-emitting element 3.

FIG. 29 shows current density-luminance characteristics of alight-emitting element 4.

FIG. 30 shows voltage-luminance characteristics of the light-emittingelement 4.

FIG. 31 shows luminance-current efficiency characteristics of thelight-emitting element 4.

FIG. 32 shows voltage-current characteristics of the light-emittingelement 4.

FIG. 33 shows the emission spectrum of the light-emitting element 4.

FIGS. 34A and 34B show the reliability of the light-emitting element 4.

FIGS. 35A and 35B are ¹H-NMR charts of a dibenzo[f,h]quinoxalinederivative represented by the structural formula (122).

FIG. 36 shows current density-luminance characteristics of alight-emitting element 5, a comparative light-emitting element 6, and acomparative light-emitting element 7.

FIG. 37 shows voltage-luminance characteristics of the light-emittingelement 5, the comparative light-emitting element 6, and the comparativelight-emitting element 7.

FIG. 38 shows luminance-current efficiency characteristics of thelight-emitting element 5, the comparative light-emitting element 6, andthe comparative light-emitting element 7.

FIG. 39 shows voltage-current characteristics of the light-emittingelement 5, the comparative light-emitting element 6, and the comparativelight-emitting element 7.

FIG. 40 shows the emission spectra of the light-emitting element 5, thecomparative light-emitting element 6, and the comparative light-emittingelement 7.

FIGS. 41A and 41B show the reliability of each of the light-emittingelement 5, the comparative light-emitting element 6, and the comparativelight-emitting element 7.

FIG. 42 shows current density-luminance characteristic of alight-emitting element 8.

FIG. 43 shows voltage-luminance characteristic of the light-emittingelement 8.

FIG. 44 shows luminance-current efficiency characteristics of thelight-emitting element 8.

FIG. 45 shows voltage-current characteristics of the light-emittingelement 8.

FIG. 46 shows the emission spectrum of the light-emitting element 8.

FIG. 47 shows voltage-current characteristics of a light-emittingelement 1A after a preservation test.

FIG. 48 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 1A after the preservation test.

FIG. 49 shows voltage-current characteristics of a light-emittingelement 2A after a preservation test.

FIG. 50 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 2A after the preservation test.

FIG. 51 shows voltage-current characteristics of a comparativelight-emitting element 3A after a preservation test.

FIG. 52 shows luminance-external quantum efficiency characteristics ofthe comparative light-emitting element 3A after the preservation test.

FIG. 53 shows voltage-current characteristics of a light-emittingelement 4A after a preservation test.

FIG. 54 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 4A after the preservation test.

FIG. 55 shows voltage-current characteristics of a light-emittingelement 8A after a preservation test.

FIG. 56 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 8A after the preservation test.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description, and themodes and details thereof can be modified in various ways withoutdeparting from the spirit and scope of the present invention.Accordingly, the present invention should not be interpreted as beinglimited to the content of the embodiments below.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, a light-emitting element which is one embodiment ofthe present invention will be described.

In a light-emitting element described in this embodiment, an EL layerincluding a light-emitting layer is provided between a pair ofelectrodes (a first electrode (anode) and a second electrode (cathode)),and the EL layer includes a hole-injection layer, a hole-transportlayer, an electron-transport layer, an electron-injection layer, and thelike in addition to the light-emitting layer.

When a voltage is applied to the light-emitting element, holes injectedfrom the first electrode side and electrons injected from the secondelectrode side recombine in the light-emitting layer; with energygenerated by the recombination, a light-emitting substance contained inthe light-emitting layer emits light.

As shown in FIG. 1, a light-emitting layer 100 contains a first organiccompound (h) 101 having an electron-transport property and ahole-transport property, a second organic compound (a) 102 having ahole-transport property, and a light-emitting substance (not shown inthe drawing). Note that the combination of the first organic compound(h) 101 and the second organic compound (a) 102 forms an excited complex(also referred to as an exciplex). That is, the lowest unoccupiedmolecular orbital (LUMO) level of the first organic compound (h) 101 isat least lower than the LUMO level of the second organic compound (a)102, and the highest occupied molecular orbital (HOMO) level of thefirst organic compound (h) 101 is at least lower than the HOMO level ofthe second organic compound (a) 102. Accordingly, as shown in thedrawing, the excitation energy of the generated exciplex is influencedby an energy difference (i.e., ΔE_(e) in the drawing) between the LUMOlevel of the first organic compound (h) 101 (LUMO (h)) and the HOMOlevel of the second organic compound (a) 102 (HOMO (a)).

Such a light-emitting layer 100 enables energy transfer utilizing anoverlap between the emission spectrum of the exciplex and the absorptionspectrum of the light-emitting substance (guest material), leading to ahigh energy transfer efficiency; thus, a light-emitting element with ahigh external quantum efficiency can be achieved. Furthermore, in orderthat the exciplex is electrically excited, electric energy (i.e.,voltage) corresponding to ΔE_(e) is needed. This energy ΔE_(e) is lowerthan energy ΔE_(h) needed for electrically exciting the first organiccompound (h) 101 and energy ΔE_(a) needed for electrically exciting thesecond organic compound (a) 102. In other words, with such alight-emitting layer 100, drive voltage (emission start voltage) of thelight-emitting element can be reduced.

Without using the first organic compound (h) 101 and the second organiccompound (a) 102, one kind of organic compound having a HOMO-LUMO gapcorresponding to ΔE_(e) may be used for a light-emitting layer, wherebydrive voltage (emission start voltage) that is low as in the case of thelight-emitting layer 100 can be achieved. However, in the case of usingone kind of organic compound, triplet excitation energy is significantlydecreased as compared to singlet excitation energy; thus, it isdifficult to achieve light emission by transfer of triplet excitationenergy to a light-emitting substance (guest material). On the otherhand, in the exciplex, singlet excitation energy and triplet excitationenergy are located at substantially the same level, which makes itpossible to transfer both singlet excitation energy and tripletexcitation energy to the light-emitting substance. As a result, as wellas the above-described effect of reduction in voltage, a higherefficiency can be achieved in the light-emitting element. A detailedmechanism of the higher efficiency will be described below.

When the light-emitting substance is a phosphorescent compound, singletexcitation energy and triplet excitation energy of the exciplex are bothtransferred to the triplet excited state of the phosphorescent compound,and light emission from the triplet excited state (i.e.,phosphorescence) is achieved; thus, the light-emitting substance is mostpreferably a phosphorescent compound in order to achieve a higherefficiency.

When the light-emitting substance is a thermally activated delayedfluorescent compound, singlet excitation energy of the exciplex istransferred to the singlet excited state of the light-emittingsubstance, and light emission from the singlet excited state (i.e.,fluorescence) is achieved. In addition, triplet excitation energy of theexciplex is transferred to the triplet excited state of thelight-emitting substance; however, reverse intersystem crossing frompart of or the entire triplet excited state to the singlet excited stateof the light-emitting substance occurs due to thermal activation, fromwhich fluorescence is achieved, leading to a higher efficiency.

When the light-emitting substance is a fluorescent compound, singletexcitation energy of the exciplex is transferred to the singlet excitedstate of the light-emitting substance, and light emission from thesinglet excited state (i.e., fluorescence) is achieved. On the otherhand, triplet excitation energy of the exciplex is transferred to thetriplet excited state of the light-emitting substance and thermallydeactivated; thus, it seems that a higher efficiency cannot be achieved.However, the exciplex that is an energy donor has a small differencebetween singlet excitation energy and triplet excitation energy and thusemits thermally activated delayed fluorescence from itself. In otherwords, in the exciplex, reverse intersystem crossing from part of or theentire triplet excited state to the singlet excited state occurs, sothat the proportion of singlet excitons is higher than that in thenormal situation. The proportion of singlet excitons in the exciplexthat is an energy donor is high, and singlet excitation energy of theexciplex is transferred to the singlet excited state of thelight-emitting substance, whereby the emission efficiency is high evenin the case of using a fluorescent compound as a light-emittingsubstance. This phenomenon is also one feature of the present invention.

As described above, a light-emitting element in which an exciplex servesas an energy donor in a light-emitting layer is effective in all thecases where a phosphorescent compound, a thermally activated delayedfluorescent compound, and a fluorescent compound are used as alight-emitting substance; however, there is a problem in controlling alight-emitting region.

As already described above, in order to form the exciplex of the firstorganic compound (h) 101 and the second organic compound (a) 102, thefollowing condition is needed: the LUMO level of the first organiccompound (h) 101 (LUMO(h)) is at least lower than the LUMO level of thesecond organic compound (a) 102 (LUMO(a)), and the HOMO level of thefirst organic compound (h) 101 (HOMO(h)) is at least lower than the HOMOlevel of the second organic compound (a) 102 (HOMO(a)). In particular,in a conventional light-emitting element in which an exciplex serves asan energy donor in the light-emitting layer 100, an energy differenceΔE_(HOMO) between the HOMO level of the first organic compound (h) 101(HOMO(h)) and the HOMO level of the second organic compound (a) 102(HOMO(a)) is made very large, thereby forming an exciplex. For example,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) is used as the first organic compound (h)101 andN-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF) is used as the second organic compound(a) 102; in that case, the HOMO level of the first organic compound (h)101 (HOMO(h)) is −6.22 eV, the HOMO level of the second organic compound(a) 102 (HOMO(a)) is −5.36 eV, and ΔE_(HOMO) is as large as 0.86 eV.

When a difference (ΔE_(HOMO)) between the HOMO level of the firstorganic compound (h) 101 (HOMO(h)) and the HOMO level of the secondorganic compound (a) 102 (HOMO(a)) is large as described above, thecarrier balance tends to be significantly changed depending on theamount of the second organic compound (a) 102 in the light-emittinglayer 100. That is, a too small amount of the second organic compound(a) 102 results in excessive electrons, and a light-emitting regionexists mainly on the anode side; on the other hand, the amount of thesecond organic compound (a) 102, which only slightly exceeds the optimalamount, results in excessive holes, and the holes are transported to thecathode side. In such a device with a narrow margin, even when the firstorganic compound (h) 101 and the second organic compound (a) 102 aremixed at an optimized ratio, a small change in the carrier balancethrough long-time driving leads to a reduction in recombinationefficiency and a lower luminance. When ΔE_(HOMO) is large, holes arestored in the second organic compound (a) 102, so that the recombinationregion in the light-emitting layer 100 is narrowed. In the case wherethe recombination region in the light-emitting layer 100 is wide, theentire light-emitting layer 100 can be used and the reliability is high.

In the light-emitting element, an electron-injection property isdecreased owing to deterioration of an electron-injection electrode(cathode) or the like, which might cause the recombination region to beshifted to the cathode side through long-time driving. In that case, theaverage distance for transporting holes to the recombination region getslonger; thus, when the light-emitting layer 100 has a poorhole-transport property, the resistance of the light-emitting element isincreased. In other words, in the case where the element is driven atconstant current, drive voltage is significantly increased over time.When the difference (ΔE_(HOMO)) between the HOMO level of the firstorganic compound (h) (HOMO(h)) and the HOMO level of the second organiccompound (a) (HOMO(a)) is large, holes are transported with difficultythrough the light-emitting layer 100; thus, an increase in drive voltageover time is very problematic.

One embodiment of the present invention solves a problem in theabove-described light-emitting element in which the exciplex serves asan energy donor in the light-emitting layer 100. That is, alight-emitting element which is one embodiment of the present inventionincludes an EL layer between an anode and a cathode. The EL layerincludes the light-emitting layer 100; the light-emitting layer 100contains the first organic compound (h) 101 having an electron-transportproperty and a hole-transport property, the second organic compound (a)102 having a hole-transport property, and the light-emitting substance;the combination of the first organic compound (h) 101 and the secondorganic compound (a) 102 forms an exciplex; the HOMO level of the firstorganic compound (h) (HOMO(h)) is lower than the HOMO level of thesecond organic compound (a) (HOMO(a)); and a difference between the HOMOlevel of the first organic compound (h) (HOMO(h)) and the HOMO level ofthe second organic compound (a) (HOMO(a)) is less than or equal to 0.4eV.

With such a structure, holes are injected not only into the secondorganic compound (a) 102 but also partly into the first organic compound(h) 101. As a result, holes are unlikely to be stored in the secondorganic compound (a) 102; thus, a light-emitting element can beobtained, in which the carrier balance can be easily maintained and therecombination region in the light-emitting layer 100 is wide. Inaddition, an increase in voltage through long-time driving (driving atconstant current) can be suppressed. In the above case, in part of thelight-emitting layer 100, recombination is caused in the first organiccompound (h) 101, which leads to the formation of an excited state ofthe first organic compound, but this is rapidly converted into anexciplex; thus, a higher efficiency can be achieved by using theexciplex. Furthermore, since holes are mainly injected into the secondorganic compound (a) 102, the effect of reducing drive voltage (emissionstart voltage) can be maintained.

In this way, ΔE_(HOMO) is set to be less than or equal to 0.4 eV (andgreater than 0 eV), and the combination of the first organic compound(h) 101 and the second organic compound (a) 102 forms an exciplex,whereby the above-described problem can be solved. Since holes areinjected not only into the second organic compound (a) 102 but also intothe first organic compound (h) 101, ΔE_(HOMO) is preferably less than orequal to 0.3 eV.

Compounds that are suitable to achieve the above concept are as follows.Preferably, the first organic compound (h) 101 includes a 6-memberednitrogen-containing heteroaromatic ring and a carbazole skeleton anddoes not include a triarylamine skeleton. That is, a compound having anelectron-transport property due to including a 6-memberednitrogen-containing heteroaromatic ring and a moderate hole-transportproperty due to including a carbazole skeleton and not including atriarylamine skeleton is preferably used. The second organic compound(a) 102 has a hole-transport property and preferably includes atriarylamine skeleton so that the HOMO level of the second organiccompound (a) 102 is higher than that of the first organic compound (h)101.

A compound including a triarylamine skeleton has, in many cases, a HOMOlevel of about −5.5 eV, or higher than or equal to −5.5 eV, on the basisof a cyclic voltammetry (CV) measurement, whereas the HOMO level of asimple 9-phenylcarbazole is −5.88 eV, so that the difference between theHOMO level of the compound including a triarylamine skeleton and that of9-phenylcarbazole is greater than or equal to 0.4 eV in many cases.Therefore, in one embodiment of the present invention, the first organiccompound (h) 101 preferably includes a bicarbazole skeleton as acarbazole skeleton, because the HOMO level of bicarbazole is higher thanthat of 9-phenylcarbazole. In particular, in one embodiment of thepresent invention, a 3,3′-bicarbazole skeleton or a 2,3′-bicarbazoleskeleton is preferably introduced into the first organic compound (h)101 because its HOMO level becomes about −5.6 eV to −5.7 eV.

Examples of the 6-membered nitrogen-containing heteroaromatic ringinclude, as well as pyridine, diazine such as pyrazine, pyrimidine, orpyridazine, triazine, and tetrazine. Such a 6-memberednitrogen-containing heteroaromatic ring may further be condensed with abenzene ring or the like. Examples of the 6-membered nitrogen-containingheteroaromatic ring condensed with a benzene ring include quinoline,isoquinoline, and dibenzo[f,h]quinoline. Furthermore, naphthyridinetypified by quinoxaline, quinazoline, and phthalazine,dibenzo[f,h]quinoxaline, dibenzo[f,h]quinazoline, and the like can alsobe used.

In one embodiment of the present invention, holes are preferablyinjected and transported not only to the second organic compound (a) 102but also to the first organic compound (h) 101, as described above.Therefore, in order to improve a hole-injection property with respect toboth the first organic compound (h) 101 and the second organic compound(a) 102 in the light-emitting layer 100, preferably, a third organiccompound (p) 105 having a hole-transport property is used for ahole-transport layer 104 in contact with the light-emitting layer 100,and the HOMO level of the third organic compound (p) 105 (HOMO(p)) isset to be lower than the HOMO level of the second organic compound (a)102 (HOMO(a)), as shown in FIG. 2. In particular, the third organiccompound (p) 105 is preferably selected so that the HOMO level of thethird organic compound (p) 105 (HOMO(p)) can be a level between the HOMOlevel of the second organic compound (a) 102 (HOMO(a)) and the HOMOlevel of the first organic compound (h) 101 (HOMO(h)).

A specific example of a light-emitting element which is one embodimentof the present invention and has the above structure will be describedbelow with reference to FIGS. 3A and 3B.

For a first electrode (anode) 201 and a second electrode (cathode) 203,a metal, an alloy, an electrically conductive compound, a mixturethereof, and the like can be used. Specifically, indium oxide-tin oxide(indium tin oxide), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti) can be used.In addition, an element belonging to Group 1 or 2 of the periodic table,for example, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as calcium (Ca) or strontium (Sr), magnesium(Mg), an alloy containing such an element (MgAg, AlLi), a rare earthmetal such as europium (Eu) or ytterbium (Yb), an alloy containing suchan element, graphene, and the like can be used. The first electrode(anode) 201 and the second electrode (cathode) 203 can be formed by, forexample, a sputtering method, an evaporation method (including a vacuumevaporation method), or the like.

A hole-injection layer 211 injects holes into a light-emitting layer 213through a hole-transport layer 212 having a high hole-transport propertyand contains a substance having a high hole-transport property (alsoreferred as a hole-transport compound) and an acceptor substance. Thehole-injection layer 211 contains a substance having a highhole-transport property and an acceptor substance, so that electrons areextracted from the substance having a high hole-transport property bythe acceptor substance to generate holes and the holes are injected intothe light-emitting layer 213 through the hole-transport layer 212. Thehole-transport layer 212 is formed using a substance having a highhole-transport property.

Examples of the substance having a high hole-transport property which isused for the hole-injection layer 211 and the hole-transport layer 212include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2); and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Alternatively, the following carbazolederivatives and the like can be used: 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole (abbreviation: CzPA).The substances described here are mainly substances each having a holemobility of higher than or equal to 1×10⁻⁶ cm²/Vs. Note that othersubstances may also be used as long as the substances have higherhole-transport properties than electron-transport properties.

Alternatively, high molecular compounds such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used.

Examples of the acceptor substance that is used for the hole-injectionlayer 211 include oxides of metals belonging to Groups 4 to 8 of theperiodic table. Specifically, molybdenum oxide is particularlypreferable.

The light-emitting layer 213 is a layer containing a light-emittingsubstance. When the light-emitting layer 213 has the structure shown inFIG. 1, the light-emitting layer 213 contains a first organic compoundhaving an electron-transport property and a hole-transport property,which will be described later, a second organic compound having ahole-transport property as described above, and a light-emittingsubstance. The combination of the first organic compound and the secondorganic compound forms an exciplex (also referred to as an excitedcomplex) at the time of recombination of carriers (electrons and holes)in the light-emitting layer. When the exciplex is formed in thelight-emitting layer, the fluorescence spectrum of the first organiccompound and that of the second organic compound are converted into theemission spectrum of the exciplex which is located on a longerwavelength side. Moreover, when the first organic compound and thesecond organic compound are selected in such a manner that the emissionspectrum of the exciplex largely overlaps with the absorption spectrumof a guest material, energy transfer from the singlet excited state canbe maximized. Also in the case of the triplet excited state, energytransfer from the exciplex, not a host material, is assumed to occur.

Although the combination of the first organic compound and the secondorganic compound can be determined such that an exciplex is formed, acombination of a compound which is likely to accept electrons (acompound having an electron-trapping property) and a compound which islikely to accept holes (a compound having a hole-trapping property) ispreferably employed. The first organic compound is preferably capable oftrapping (or transporting) not only electrons but also holes and thuspreferably includes a 6-membered nitrogen-containing heteroaromatic ringand a bicarbazole skeleton and does not include a triarylamine skeleton.For example, a compound represented by the following general formula(G0) is preferably used.

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group.

When Ar includes an anthracenylene group, triplet excitation energy ofthe compound is largely decreased (to an energy of lower than or equalto 1.7 eV), in which case triplet excitation energy of the exciplexmight be quenched. Therefore, preferably, an arylene group in Ar doesnot include an anthracenylene group.

Specifically, the compounds represented by the general formulae (G1) to(G3) are preferably used. More specifically,2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo i[h]quinoxaline (abbreviation: 2PCCzPDBq),2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzPDBq),2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2PCCzPDBq-02), and2-{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq-02) can be used.

Examples of the compound which is likely to accept holes includecompounds having triarylamine skeletons such as4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-N,N-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N,N′-triphenyl-N,N,N′-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N-bis[4-(carbazol-9-yl)phenyl]-N,N-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), and3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2).

The first organic compound and the second organic compound are notlimited to the above examples as long as the combination of the firstorganic compound and the second organic compound can form an exciplex,the emission spectrum of the exciplex overlaps with the absorptionspectrum of the light-emitting substance, and the peak of the emissionspectrum of the exciplex has a longer wavelength than the peak of theabsorption spectrum of the light-emitting substance.

Note that in the case where the compound which is likely to acceptelectrons and the compound which is likely to accept holes are used asthe first organic compound and the second organic compound, the carrierbalance can be controlled by the mixture ratio of the compounds.Specifically, the ratio of the first organic compound to the secondorganic compound is preferably 1:9 to 9:1.

As the materials that can be used as the light-emitting substance andthe emission center substance in the light-emitting layer 213, alight-emitting substance converting singlet excitation energy into lightemission, a light-emitting substance converting triplet excitationenergy into light emission, and the like can be used alone or incombination. Described below are examples of the light-emittingsubstance and the emission center substance.

As an example of the light-emitting substance converting singletexcitation energy into light emission, a substance which emitsfluorescence (a fluorescent compound) can be given.

Examples of the substance which emits fluorescence includeN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,I-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine] (abbreviation:DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N,N-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD),{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM), and2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM).

Examples of the light-emitting substance converting triplet excitationenergy into light emission include a substance which emitsphosphorescence (a phosphorescent compound) and a thermally activateddelayed fluorescent (TADF) material which emits thermally activateddelayed fluorescence. Note that “delayed fluorescence” exhibited by theTADF material refers to light emission having the same spectrum asnormal fluorescence and an extremely long lifetime. The lifetime is 10⁻⁶seconds or longer, preferably 10⁻³ seconds or longer.

Examples of the substance which emits phosphorescence include bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac),tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:Ir(ppy)₂(acac)), tris(acetylacetonato) (monophenanthroline)terbium(III)(abbreviation: Tb(acac)₃(Phen)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(bt)₂(acac)),bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]),(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-iPr)₂(acac)]),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)],(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP), tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)).

Examples of the TADF material include fullerene, a derivative thereof,an acridine derivative such as proflavine, and eosin. Other examplesinclude a metal-containing porphyrin, such as a porphyrin containingmagnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium(In), or palladium (Pd). Examples of the metal-containing porphyrininclude a protoporphyrin-tin fluoride complex (SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂OEP). Alternatively,a heterocyclic compound including a π-electron rich heteroaromatic ringand a π-electron deficient heteroaromatic ring can be used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(PIC-TRZ). Note that a substance in which the π-electron richheteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because both a donorproperty of the π-electron rich heteroaromatic ring and an acceptorproperty of the π-electron deficient heteroaromatic ring are increasedand the energy difference between the S₁ level and the T₁ level becomessmall.

The light-emitting layer 213 may have a stacked structure as illustratedin FIG. 3B. In that case, each layer in the stacked structure emitslight. For example, fluorescence is obtained from a first light-emittinglayer 213 (a1), and phosphorescence is obtained from a secondlight-emitting layer 213 (a2) stacked over the first layer. Note thatthe stacking order may be reversed. It is preferable that light emissiondue to energy transfer from an exciplex to a dopant be obtained from thelayer that emits phosphorescence. In the case where blue light emissionis obtained from one of the first and second layers, orange or yellowlight emission can be obtained from the other layer. Each layer may alsocontain plural kinds of dopants.

An electron-transport layer 214 is a layer containing a substance havinga high electron-transport property (also referred to as anelectron-transport compound). For the electron-transport layer 214, ametal complex such as tris(8-quinolinolato)aluminum(III) (abbreviation:Alq₃), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation:Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation:BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis[2-2(hydroxyphenyl)benzoxazolato]zinc(II)(abbreviation: Zn(BOX)₂), orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation: Zn(BTZ)₂)can be used. Alternatively, it is possible to use a heteroaromaticcompound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Furtheralternatively, it is possible to use a high molecular compound such aspoly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy). The substances mentioned here are mainlysubstances each having an electron mobility of higher than or equal to1×10⁻⁶ cm²/Vs. Note that other substances may also be used for theelectron-transport layer 214 as long as the substances have higherelectron-transport properties than hole-transport properties.

The electron-transport layer 214 is not limited to a single layer andmay have a stacked structure including two or more layers containing anyof the above substances.

An electron-injection layer 215 is a layer containing a substance havinga high electron-injection property. The electron-injection layer 215 canbe formed using an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)). Alternatively, a rare earthmetal compound such as erbium fluoride (ErF₃) can be used.Alternatively, an electride may be used for the electron-injection layer215. Examples of the electride include a substance in which electronsare added at high concentration to calcium oxide-aluminum oxide. Any ofthe substances for forming the electron-transport layer 214, which aregiven above, can also be used.

The electron-injection layer 215 may also be formed using a compositematerial in which an organic compound and an electron donor are mixed.The composite material is superior in an electron-injection property andan electron-transport property, because electrons are generated in theorganic compound by the electron donor. In that case, as the organiccompound, a material that can efficiently transport the producedelectrons is preferably used; for example, any of the above-describedsubstances that are used to form the electron-transport layer 214 (e.g.,a metal complex or a heteroaromatic compound) can be used. As theelectron donor, a substance showing an electron-donating property withrespect to the organic compound may be used. Specifically, an alkalimetal, an alkaline earth metal, and a rare earth metal are preferable,and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the likecan be given. Furthermore, an alkali metal oxide or an alkaline earthmetal oxide is preferable, and for example, lithium oxide, calciumoxide, barium oxide, and the like can be given. Alternatively, Lewisbase such as magnesium oxide can be used. An organic compound such astetrathiafulvalene (abbreviation: TTF) can also be used.

Note that the hole-injection layer 211, the hole-transport layer 212,the light-emitting layer 213, the electron-transport layer 214, and theelectron-injection layer 215, which are mentioned above, can each beformed by a method, such as an evaporation method (including a vacuumevaporation method), an inkjet method, or a coating method.

In the above-described light-emitting element, holes and electrons arerecombined in the EL layer 202, whereby light is emitted. This emittedlight is extracted out through the first electrode 201 and/or the secondelectrode 203. Therefore, the first electrode 201 and/or the secondelectrode 203 is an electrode having a light-transmitting property.

The light-emitting element described in this embodiment enables energytransfer utilizing an overlap between the emission spectrum of theexciplex and the absorption spectrum of a phosphorescent compound (guestmaterial), leading to a high energy transfer efficiency; thus, alight-emitting element with a high external quantum efficiency can beachieved.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, a dibenzo[f,h]quinoxaline derivative which can beused in a light-emitting element and is one embodiment of the presentinvention will be described.

A dibenzo[f,h]quinoxaline derivative which is one embodiment of thepresent invention is represented by the following general formula (G0).

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group.

The dibenzo[f,h]quinoxaline derivative represented by the generalformula (G0) can be synthesized by the following synthesis method. Asshown in the following synthesis scheme (a), the dibenzo[f,h]quinoxalinederivative represented by the general formula (G0) can be obtained byreacting a halogen compound (A1) of a dibenzo[f,h]quinoxaline derivativewith an arylboronic acid compound (A2) of a bicarbazole derivative.

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group. Furthermore, X represents a halogen. When Ar is asubstituted or unsubstituted arylene group having 6 to 25 carbon atoms,B represents a boronic acid, a boronic ester, a cyclic-triolborate salt,or the like. As the cyclic-triolborate salt, a lithium salt, a potassiumsalt, or a sodium salt may be used. When Ar is a single bond, Brepresents hydrogen.

In addition, as shown in the following synthesis scheme (b), thedibenzo[f,h]quinoxaline derivative represented by the general formula(G0) can be obtained in such a manner that an intermediate (B2) isobtained through a reaction of a halogen compound (A1) of adibenzo[f,h]quinoxaline derivative with a halogen-substitutedarylboronic acid (B1) and then made to react with a bicarbazolederivative (B3).

In the formula, A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond. Preferably, an arylene group in Ar does not include ananthracenylene group. Furthermore, X represents a halogen. B representsa boronic acid, a boronic ester, a cyclic-triolborate salt, or the like.

When Ar in the general formula (G0) is a single bond, a halogen compound(A1) of a dibenzo[f,h]quinoxaline derivative is directly reacted with abicarbazole derivative (B3).

A dibenzo[f,h]quinoxaline derivative which is one embodiment of thepresent invention and can be synthesized by any of the above synthesismethods is preferably a compound represented by any of the above generalformulae (G1) to (G3). Specific structural formulae ofdibenzo[f,h]quinoxaline derivatives which are embodiments of the presentinvention and represented by the general formulae (G0) to (G3) are shownbelow (the following structural formulae (100) to (131)). Note that thepresent invention is not limited thereto.

A dibenzo[f,h]quinoxaline derivative which is one embodiment of thepresent invention is used in a light-emitting element of one embodimentof the present invention, whereby a light-emitting element, alight-emitting device, an electronic device, or a lighting device with ahigh emission efficiency and a high reliability can be obtained. It isalso possible to achieve a light-emitting element, a light-emittingdevice, an electronic device, or a lighting device with low powerconsumption.

The dibenzo[f,h]quinoxaline derivative represented by any of the generalformulae (G0) to (G3) has an electron-transport property and ahole-transport property and thus can be used as a host material of alight-emitting layer or used for an electron-transport layer or ahole-transport layer. Furthermore, the dibenzo[f,h]quinoxalinederivative represented by any of the general formulae (G0) to (G3) emitsfluorescence and thus can be used as a light-emitting substance of alight-emitting element. As described above, the dibenzo[f,h]quinoxalinederivative represented by any of the general formulae (G0) to (G3) is auseful novel compound which can be used as various materials in alight-emitting element; thus, a light-emitting element containing thedibenzo[f,h]quinoxaline derivative represented by any of the generalformulae (G0) to (G3) is a light-emitting element which is oneembodiment of the present invention.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, as a light-emitting element which is one embodimentof the present invention, a light-emitting element (hereinafter referredto as a tandem light-emitting element) that includes a plurality of ELlayers with a charge-generation layer provided therebetween will bedescribed. As illustrated in FIG. 4A, the tandem light-emitting elementincludes a plurality of EL layers (a first EL layer 402(1) and a secondEL layer 402(2)) between a pair of electrodes (a first electrode 401 anda second electrode 404).

In this embodiment, the first electrode 401 functions as an anode, andthe second electrode 404 functions as a cathode. The first electrode 401and the second electrode 404 can have structures similar to thosedescribed in Embodiment 1. All or any of the plurality of EL layers (thefirst EL layer 402(1) and the second EL layer 402(2)) may havestructures similar to those described in Embodiment 1. In other words,the structures of the first EL layer 402(1) and the second EL layer402(2) may be the same or different from each other and can be similarto those described in Embodiment 1. The dibenzo[f,h]quinoxalinederivative described in Embodiment 2 can be used for any of theplurality of EL layers (the first EL layer 402(1) and the second ELlayer 402(2)).

A charge-generation layer 405 is provided between the plurality of ELlayers (the first EL layer 402(1) and the second EL layer 402(2)). Thecharge-generation layer 405 has a function of injecting electrons intoone of the EL layers and injecting holes into the other of the EL layerswhen a voltage is applied between the first electrode 401 and the secondelectrode 404. In this embodiment, when a voltage is applied such thatthe potential of the first electrode 401 is higher than that of thesecond electrode 404, the charge-generation layer 405 injects electronsinto the first EL layer 402(1) and injects holes into the second ELlayer 402(2).

In terms of light extraction efficiency, the charge-generation layer 405preferably has a light-transmitting property with respect to visiblelight (specifically, the charge-generation layer 405 has a visible lighttransmittance of higher than or equal to 40%). Furthermore, thecharge-generation layer 405 functions even if it has lower conductivitythan the first electrode 401 or the second electrode 404.

The charge-generation layer 405 may have either a structure in which anelectron acceptor is added to an organic compound having a highhole-transport property or a structure in which an electron donor isadded to an organic compound having a high electron-transport property.Alternatively, both of these structures may be stacked.

In the case of the structure in which an electron acceptor is added toan organic compound having a high hole-transport property, as theorganic compound having a high hole-transport property, for example, anaromatic amine compound such as NPB, TPD, TDATA, MTDATA, or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), or the like can be used. The substances describedhere are mainly substances each having a hole mobility of higher than orequal to 1×10⁻⁶ cm²/Vs. Note that other substances may also be used aslong as the substances have higher hole-transport properties thanelectron-transport properties.

As the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Moreover, oxides of metals belonging toGroups 4 to 8 of the periodic table can be given. Specifically, it ispreferable to use vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide because of their high electron accepting properties. Inparticular, molybdenum oxide is preferable because of its stability inthe air, a low hygroscopic property, and easiness of handling.

On the other hand, in the case of the structure in which an electrondonor is added to an organic compound having a high electron-transportproperty, as the organic compound having a high electron-transportproperty, for example, a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as Alq₃, Almq₃, BeBq₂, or BAlq, or thelike can be used. Alternatively, a metal complex having an oxazole-basedligand or a thiazole-based ligand, such as Zn(BOX)₂ or Zn(BTZ)₂ can beused. Alternatively, in addition to such a metal complex, PBD, OXD-7,TAZ, BPhen, BCP, or the like can be used. The substances described hereare mainly substances each having an electron mobility of higher than orequal to 1×10⁻⁶ cm²/Vs. Note that other substances may also be used aslong as the substances have higher electron-transport properties thanhole-transport properties.

As the electron donor, it is possible to use an alkali metal, analkaline earth metal, a rare earth metal, a metal belonging to Group 2or 13 of the periodic table, or an oxide or carbonate thereof.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the electron donor.

Note that forming the charge-generation layer 405 by using any of theabove materials can suppress an increase in drive voltage caused by thestack of the EL layers.

Although the light-emitting element including two EL layers is describedin this embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers (402(1) to 402(n)) (n isthree or more) are stacked as illustrated in FIG. 4B. In the case wherea plurality of EL layers are included between a pair of electrodes as inthe light-emitting element of this embodiment, by providingcharge-generation layers (405(1) to 405(n−1)) between the EL layers,light emission in a high luminance region can be obtained with currentdensity kept low. Since the current density can be kept low, the elementcan have a long lifetime. In addition, when the light-emitting elementis applied to a light-emitting device, an electronic device, a lightingdevice, and the like each having a large light-emitting area, voltagedrop due to resistance of an electrode material can be reduced, therebyachieving homogeneous light emission in a large area.

When the EL layers have different emission colors, a desired emissioncolor can be obtained from the whole light-emitting element. Forexample, in the light-emitting element including two EL layers, when anemission color of a first EL layer and an emission color of a second ELlayer are made to be complementary colors, a light-emitting elementemitting white light as a whole light-emitting element can be obtained.Note that “complementary color” means a relation between colors whichbecomes an achromatic color when they are mixed. In other words, whenlights which are complementary to each other are mixed, white lightemission can be obtained. Specifically, a combination in which bluelight emission is obtained from the first EL layer and yellow (ororange) light emission is obtained from the second EL layer is given asan example. In that case, it is not necessary that blue light emissionand yellow (or orange) light emission are both fluorescence or bothphosphorescence. For example, a combination in which blue light emissionis fluorescence and yellow (or orange) light emission is phosphorescenceor a combination in which blue light emission is phosphorescence andyellow (or orange) light emission is fluorescence may be employed.Moreover, a stacked structure suitable for adjustment of an optical pathlength of the light-emitting element (e.g., a structure in which a firstlight-emitting layer exhibits yellow light emission and a secondlight-emitting layer exhibits blue light emission) is preferablyemployed, in which case the element characteristics can be furtherimproved.

Also in a light-emitting element including three EL layers, for example,white light emission can be similarly obtained from a wholelight-emitting element when an emission color of a first EL layer isred, an emission color of a second EL layer is green, and an emissioncolor of a third EL layer is blue.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, one embodiment of a light-emitting device in whichthe light-emitting element described in Embodiment 1 is combined with acoloring layer (a color filter or the like) will be described. In thisembodiment, the structure of a pixel portion of the light-emittingdevice will be described with reference to FIG. 5.

In FIG. 5, a plurality of FETs (transistors) 502 are formed over asubstrate 501. Each of the FETs 502 is electrically connected to alight-emitting element (507R, 507G, 507B, or 507Y). Specifically, eachof the FETs 502 is electrically connected to a first electrode 503 thatis a pixel electrode of the light-emitting element. A partition wall 504is provided to cover end portions of the adjacent first electrodes 503.

The first electrode 503 in this embodiment has a function of areflective electrode. An EL layer 505 is formed over the first electrode503, and a second electrode 510 is formed over the EL layer 505. The ELlayer 505 includes a plurality of light-emitting layers each emittingmonochromatic light. The second electrode 510 has a function of asemi-transmissive and semi-reflective electrode.

The light-emitting elements (507R, 507G, 507B, and 507Y) emit light ofdifferent colors. Specifically, the light-emitting element 507R isoptically adjusted to emit red light, and in a region indicated by 506R,red light is emitted through a coloring layer 508R in the directionindicated by an arrow. The light-emitting element 507G is opticallyadjusted to emit green light, and in a region indicated by 506G, greenlight is emitted through a coloring layer 508G in the directionindicated by an arrow. The light-emitting element 507B is opticallyadjusted to emit blue light, and in a region indicated by 506B, bluelight is emitted through a coloring layer 508B in the directionindicated by an arrow. The light-emitting element 507Y is opticallyadjusted to emit yellow light, and in a region indicated by 506Y, yellowlight is emitted through a coloring layer 508Y in the directionindicated by an arrow.

As illustrated in FIG. 5, each of the coloring layers (508R, 508G, 508B,and 508Y) is provided on a transparent sealing substrate 511 that isprovided above the substrate 501 over which the light emitting elements(507R, 507G, 507B, and 507Y) are formed. Note that the coloring layers(508R, 508G, 508B, and 508Y) are provided in positions overlapping withthe corresponding light-emitting elements (507R, 507G, 507B, and 507Y)which exhibit different emission colors.

A black layer (black matrix) 509 is provided to overlap with endportions of the adjacent coloring layers (508R, 508G, 508B, and 508Y).Note that the coloring layers (508R, 508G, 508B, and 508Y) and the blacklayer 509 may be covered with an overcoat layer that is formed using atransparent material.

The above-described light-emitting device has a structure in which lightis extracted from the sealing substrate 511 side (a top emissionstructure), but may have a structure in which light is extracted fromthe substrate 501 side where the FETs are formed (a bottom emissionstructure). Note that in the light-emitting device having a top emissionstructure described in this embodiment, a light-blocking substrate or alight-transmitting substrate can be used as the substrate 501, whereasin a light-emitting device having a bottom emission structure, alight-transmitting substrate needs to be used as the substrate 501.

In this specification and the like, a transistor or a light-emittingelement can be formed using any of a variety of substrates, for example.The type of a substrate is not limited to a certain type. As thesubstrate, a semiconductor substrate (e.g., a single crystal substrateor a silicon substrate), an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a metal substrate, a stainless steelsubstrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, anattachment film, paper including a fibrous material, a base materialfilm, or the like can be used, for example. Examples of a glasssubstrate include a barium borosilicate glass substrate, analuminoborosilicate glass substrate, and a soda lime glass substrate.Examples of the flexible substrate, the attachment film, the basematerial film, and the like include plastics typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone(PES), and polytetrafluoroethylene (PTFE), a synthetic resin such asacrylic, films formed of polypropylene, polyester, polyvinyl fluoride,and polyvinyl chloride, films formed of polyamide, polyimide, aramid,and epoxy, an inorganic film formed by evaporation, and paper.Specifically, when a transistor is fabricated using a semiconductorsubstrate, a single crystal substrate, an SOI substrate, or the like, atransistor with few variations in characteristics, size, shape, or thelike, high current supply capability, and a small size can befabricated. A circuit using such a transistor achieves lower powerconsumption or higher integration.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor or the light-emitting element may be provided directlyover the flexible substrate. Further alternatively, a separation layermay be provided between a substrate and the transistor or the like. Theseparation layer can be used when part of or the entire semiconductordevice formed over the separation layer is separated from the substrateand transferred onto another substrate. In such a case, the transistoror the like can be transferred to a substrate having low heat resistanceor a flexible substrate as well. For the above separation layer, a stackincluding inorganic films, a tungsten film and a silicon oxide film, oran organic resin film of polyimide or the like formed over a substratecan be used, for example.

In other words, after the transistor or the light-emitting element isformed using one substrate, the transistor or the light-emitting elementmay be transferred to another substrate. Examples of a substrate towhich the transistor or the light-emitting element is transferredinclude, in addition to the above-described substrates over which atransistor or the like can be formed, a paper substrate, a cellophanesubstrate, an aramid film substrate, a polyimide film substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon,polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra,rayon, or regenerated polyester), or the like), a leather substrate, anda rubber substrate. When such a substrate is used, a transistor or thelike with excellent properties or low power consumption can be formed, adevice with high durability and high heat resistance can be provided, ora reduction in weight or thickness can be achieved.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, a light-emitting device including a light-emittingelement in which a dibenzo[f,h]quinoxaline derivative which is oneembodiment of the present invention is used for an EL layer will bedescribed.

The light-emitting device may be either a passive matrix light-emittingdevice or an active matrix light-emitting device. Note that any of thelight-emitting elements described in the other embodiments can beapplied to a light-emitting device described in this embodiment.

In this embodiment, an active matrix light-emitting device will bedescribed with reference to FIGS. 6A and 6B.

FIG. 6A is a top view illustrating the light-emitting device, and FIG.6B is a cross-sectional view taken along the dashed-dotted line A-A′ inFIG. 6A. The active matrix light-emitting device of this embodimentincludes a pixel portion 602 provided over an element substrate 601, adriver circuit portion (a source line driver circuit) 603, and drivercircuit portions (gate line driver circuits) 604 a and 604 b. The pixelportion 602, the driver circuit portion 603, and the driver circuitportions 604 a and 604 b are sealed with a sealant 605 between theelement substrate 601 and a sealing substrate 606.

In addition, over the element substrate 601, a lead wiring 607 forconnecting an external input terminal, through which a signal (e.g., avideo signal, a clock signal, a start signal, a reset signal, or thelike) or potential is transmitted from the outside to the driver circuitportion 603 and the driver circuit portions 604 a and 604 b, isprovided. Here, an example is described in which a flexible printedcircuit (FPC) 608 is provided as the external input terminal. Althoughonly the FPC is illustrated here, the FPC may be provided with a printedwiring board (PWB). The light-emitting device in this specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with an FPC or a PWB.

Next, a cross-sectional structure will be described with reference toFIG. 6B. The driver circuit portions and the pixel portion are formedover the element substrate 601; here are illustrated the driver circuitportion 603 which is the source line driver circuit and the pixelportion 602.

In the driver circuit portion 603, an FET 609 and an FET 610 arecombined as an example. Note that the driver circuit portion 603 may beformed with a circuit including transistors having the same conductivitytype (either n-channel transistors or p-channel transistors) or a CMOScircuit including an n-channel transistor and a p-channel transistor. Inthis embodiment, a driver-integrated type in which a driver circuit isformed over a substrate is shown; however, the driver circuit can alsobe formed outside the substrate.

The pixel portion 602 includes a plurality of pixels each including aswitching FET 611, a current control FET 612, and a first electrode(anode) 613 electrically connected to a wiring (a source electrode or adrain electrode) of the current control FET 612. In this embodiment, thepixel portion 602 includes two FETs, the switching FET 611 and thecurrent control FET 612, but one embodiment of the present invention isnot limited thereto. The pixel portion 602 may include, for example,three or more FETs and a capacitor in combination.

As the FETs 609, 610, 611, and 612, for example, a staggered transistoror an inverted staggered transistor can be used. Examples of asemiconductor material that can be used for the FETs 609, 610, 611, and612 include Group 13 semiconductors (e.g., gallium), Group 14semiconductors (e.g., silicon), compound semiconductors, oxidesemiconductors, and organic semiconductors. In addition, there is noparticular limitation on the crystallinity of the semiconductormaterial, and an amorphous semiconductor film or a crystallinesemiconductor film can be used, for example. An oxide semiconductor ispreferably used for the FETs 609, 610, 611, and 612. Examples of theoxide semiconductor include an In—Ga oxide and an In-M-Zn oxide (M isAl, Ga, Y, Zr, La, Ce, or Nd). For example, an oxide semiconductormaterial that has an energy gap of 2 eV or more, preferably 2.5 eV ormore, more preferably 3 eV or more is used for the FETs 609, 610, 611,and 612, so that the off-state current of the transistors can bereduced.

An insulator 614 is formed to cover an end portion of the firstelectrode 613. In this embodiment, the insulator 614 is formed using apositive photosensitive acrylic resin. The first electrode 613 is usedas an anode in this embodiment.

The insulator 614 preferably has a curved surface with curvature at itsupper end portion or lower end portion. This enables the coverage with afilm to be formed over the insulator 614 to be favorable. The insulator614 can be formed using, for example, either a negative photosensitiveresin or a positive photosensitive resin. The material of the insulator614 is not limited to an organic compound, and an inorganic compoundsuch as silicon oxide, silicon oxynitride, or silicon nitride can alsobe used.

An EL layer 615 and a second electrode (cathode) 616 are stacked overthe first electrode (anode) 613. At least a light-emitting layer isprovided in the EL layer 615. Furthermore, in the EL layer 615, ahole-injection layer, a hole-transport layer, an electron-transportlayer, an electron-injection layer, a charge-generation layer, and thelike can be provided as appropriate in addition to the light-emittinglayer.

A light-emitting element 617 is formed of a stacked structure of thefirst electrode (anode) 613, the EL layer 615, and the second electrode(cathode) 616. For the first electrode (anode) 613, the EL layer 615,and the second electrode (cathode) 616, the materials described inEmbodiment 2 can be used. Although not illustrated here, the secondelectrode (cathode) 616 is electrically connected to the FPC 608 whichis an external input terminal.

Although the cross-sectional view in FIG. 6B illustrates only onelight-emitting element 617, a plurality of light-emitting elements arearranged in a matrix in the pixel portion 602. Light-emitting elementswhich emit light of three kinds of colors (R, G, and B) are selectivelyformed in the pixel portion 602, whereby a light-emitting device capableof full color display can be formed. In addition to the light-emittingelements that emit light of three kinds of colors (R, G, and B), forexample, light-emitting elements that emit light of white (W), yellow(Y), magenta (M), cyan (C), and the like may be formed. For example, thelight-emitting elements that emit light of a plurality of kinds ofcolors are used in combination with the light-emitting elements thatemit light of three kinds of colors (R, G, and B), whereby effects suchas an improvement in color purity and a reduction in power consumptioncan be obtained. Alternatively, a light-emitting device which is capableof full color display may be fabricated by a combination with colorfilters. Furthermore, the light-emitting device may have an improvedemission efficiency and a reduced power consumption by combination withquantum dots.

The sealing substrate 606 is attached to the element substrate 601 withthe sealant 605, so that the light-emitting element 617 is provided in aspace 618 surrounded by the element substrate 601, the sealing substrate606, and the sealant 605. The space 618 may be filled with an inert gas(such as nitrogen or argon) or the sealant 605.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material do not transmit moisture oroxygen as much as possible. As the element substrate 601 and the sealingsubstrate 606, a glass substrate, a quartz substrate, or a plasticsubstrate formed of fiber reinforced plastics (FRP), poly(vinylfluoride) (PVF), polyester, acrylic, or the like can be used. In thecase where glass frit is used as the sealant, the element substrate 601and the sealing substrate 606 are preferably glass substrates for highadhesion.

In the above manner, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, various examples of an electronic devicemanufactured using a light-emitting device which is one embodiment ofthe present invention will be described with reference to FIGS. 7A to7D.

Examples of an electronic device to which a light-emitting device isapplied include television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, mobile phones(also referred to as cellular phones or portable telephone devices),portable game machines, portable information terminals, audio playbackdevices, and large game machines such as pin-ball machines. Specificexamples of these electronic devices are illustrated in FIGS. 7A to 7D.

FIG. 7A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.The display portion 7103 can display images and may be a touch panel (aninput/output device) including a touch sensor (an input device). Notethat the light-emitting device which is one embodiment of the presentinvention can be used for the display portion 7103. In addition, here,the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying information output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 7B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnecting port 7205, a pointing device 7206, and the like. Note thatthis computer can be manufactured using the light-emitting device whichis one embodiment of the present invention for the display portion 7203.The display portion 7203 may be a touch panel (an input/output device)including a touch sensor (an input device).

FIG. 7C illustrates a smart watch, which includes a housing 7302, adisplay panel 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like.

The display panel 7304 mounted in the housing 7302 serving as a bezelincludes a non-rectangular display region. The display panel 7304 candisplay an icon 7305 indicating time, another icon 7306, and the like.The display panel 7304 may be a touch panel (an input/output device)including a touch sensor (an input device).

The smart watch illustrated in FIG. 7C can have a variety of functions,for example, a function of displaying a variety of information (e.g., astill image, a moving image, and a text image) on a display portion, atouch panel function, a function of displaying a calendar, date, time,and the like, a function of controlling processing with a variety ofsoftware (programs), a wireless communication function, a function ofbeing connected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading a program or data stored in a recording medium and displayingthe program or data on a display portion.

The housing 7302 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. Note that the smart watch can be manufacturedusing the light-emitting device for the display panel 7304.

FIG. 7D illustrates an example of a mobile phone (e.g., smartphone). Amobile phone 7400 includes a housing 7401 provided with a displayportion 7402, a microphone 7406, a speaker 7405, a camera 7407, anexternal connection portion 7404, an operation button 7403, and thelike. In the case where a light-emitting device is manufactured byforming the light-emitting element of one embodiment of the presentinvention over a flexible substrate, the light-emitting device can beused for the display portion 7402 with a curved surface as illustratedin FIG. 7D.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 7D is touched with a finger or the like, information can be inputinto the mobile phone. Furthermore, operations such as making a call andcreating an e-mail can be performed by touching the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating an e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device such as a gyroscope or an acceleration sensor isprovided inside the mobile phone 7400, display on the screen of thedisplay portion 7402 can be automatically changed by determining theorientation of the mobile phone 7400 (whether the mobile phone is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can also be changed depending on the kind of images displayed onthe display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is changed to the display mode. When the signal is a signalof text data, the screen mode is changed to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, when a backlightor a sensing light source which emits near-infrared light is provided inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Furthermore, the light-emitting device can also be used for a mobilephone having a structure illustrated in FIG. 7D′1 or FIG. 7D′2, which isanother structure of the mobile phone (e.g., smartphone).

In the case of the structure illustrated in FIG. 7D′1 or FIG. 7D′2,character information, image information, or the like can be displayedon second screens 7502(1) and 7502(2) of housings 7500(1) and 7500(2) aswell as first screens 7501(1) and 7501(2). Such a structure enables auser to easily see character information, image information, or the likedisplayed on the second screens 7502(1) and 7502(2) while the mobilephone is placed in user's breast pocket.

FIGS. 8A to 8C illustrate a foldable portable information terminal 9310.FIG. 8A illustrates the portable information terminal 9310 that isopened. FIG. 8B illustrates the portable information terminal 9310 thatis being opened or being folded. FIG. 8C illustrates the portableinformation terminal 9310 that is folded. The portable informationterminal 9310 is highly portable when folded. When the portableinformation terminal 9310 is opened, a seamless large display region ishighly browsable.

A display panel 9311 is supported by three housings 9315 joined togetherby hinges 9313. The display panel 9311 may be a touch panel (aninput/output device) including a touch sensor (an input device). Bybending the display panel 9311 at a connection portion between twohousings 9315 with the use of the hinges 9313, the portable informationterminal 9310 can be reversibly changed in shape from an opened state toa folded state. The light-emitting device of one embodiment of thepresent invention can be used for the display panel 9311. A displayregion in the display panel 9311 includes a display region that ispositioned at a side surface of the portable information terminal 9310that is folded. On this display region, information icons,frequently-used applications, file shortcuts to programs, and the likecan be displayed, and confirmation of information and start ofapplication can be smoothly performed.

As described above, the electronic devices can be obtained by using thelight-emitting device of one embodiment of the present invention. Notethat the light-emitting device can be used for electronic devices in avariety of fields without being limited to the electronic devicesdescribed in this embodiment.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 7

In this embodiment, examples of a lighting device to which alight-emitting device which is one embodiment of the present inventionis applied, will be described with reference to FIG. 9.

FIG. 9 illustrates an example in which the light-emitting device is usedas an indoor lighting device 8001. Since the area of the light-emittingdevice can be increased, a lighting device having a large area can beformed. In addition, with the use of a housing with a curved surface, alighting device 8002 which includes the housing, a cover, or a supportand in which a light-emitting region has a curved surface can beobtained. A light-emitting element included in the light-emitting devicedescribed in this embodiment is in a thin film form, which allows thehousing to be designed more freely. Therefore, the lighting device canbe elaborately designed in a variety of ways. Furthermore, a wall of theroom may be provided with a large-sized lighting device 8003.

Moreover, when the light-emitting device is used for a table by beingused as a surface of the table, a lighting device 8004 which has afunction as a table can be obtained. When the light-emitting device isused as part of other furniture, a lighting device which has a functionas the furniture can be obtained.

In this manner, a variety of lighting devices to which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example 1 Synthesis Example 1

In this example, as a synthesis method of one embodiment of the presentinvention, a synthesis method of2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2PCCzPDBq, represented by the structural formula (100))will be described. The structure of 2PCCzPDBq is shown below.

<Synthesis of 2PCCzPDBq>

First, 1.0 g (2.8 mmol) of 2-(4-chlorophenyl)dibenzo[f,h]quinoxaline,1.1 g (2.8 mmol) of 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.54 g(5.6 mmol) of sodium-tert-butoxide, and 23 mg (0.10 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos)were put in a 200-mL three-neck flask and mixed, and the air in theflask was replaced with nitrogen. To this mixture was added 14 mL ofmesitylene, and the resulting mixture was degassed by being stirredwhile the pressure in the flask was reduced.

Then, 16 mg (0.028 mmol) of bis(dibenzylideneacetone)palladium(0)(abbreviation: Pd(dba)₂) was added to the mixture. This mixture wasstirred at 150° C. for 5 hours under a nitrogen stream, so that a solidwas precipitated. The precipitated solid was collected by suctionfiltration. The collected solid was dissolved in approximately 400 mL ofhot toluene, and this solution was suction-filtered through a stack ofCelite and alumina. The resulting filtrate was concentrated to give asolid. The solid was recrystallized with toluene to give 1.6 g of ayellow powder, which was the target substance, in a yield of 80%.

By a train sublimation method, 1.4 g of the obtained yellow powderedsolid, which was the target substance, was purified. The sublimationpurification was carried out at 380° C. under a pressure of 3.8 Pa witha flow rate of an argon gas at 10 mL/min. After the sublimationpurification, 1.1 g of a yellow glassy solid of 2PCCzPDBq was obtainedat a collection rate of 79%. The synthesis scheme of this step is shownin the following scheme (a-1).

Analysis results by nuclear magnetic resonance spectroscopy (¹H-NMR) ofthe yellow powdered solid obtained in the above step will be describedbelow. ¹H-NMR charts are shown in FIGS. 10A and 10B. FIG. 10B is a chartin which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ)in FIG. 10A is enlarged. These results show that 2PCCzPDBq (representedby the structural formula (100)) was obtained in the above step.

δ=7.32 (t, J=5.7 Hz, 1H), 7.37 (t, J=8.0 Hz, 1H), 7.43-7.53 (m, 5H),7.58-7.66 (m, 6H), 7.72-7.89 (m, 8H), 8.25 (d, J=7.4 Hz, 1H), 8.30 (d,J=8.1 Hz, 1H), 8.44 (d, J=7.5 Hz, 1H), 8.50 (d, J=5.4 Hz, 2H), 8.64-8.67(m, 3H), 9.25 (d, J=8.0 Hz, 1H), 9.37 (d, J=6.3 Hz, 1H), 9.47 (s, 1H).

Then, FIG. 11 shows the absorption spectrum and the emission spectrum of2PCCzPDBq in a toluene solution of 2PCCzPDBq, and FIG. 12 shows theabsorption spectrum and the emission spectrum of 2PCCzPDBq in a thinfilm of 2PCCzPDBq. The spectra were measured with a UV-visiblespectrophotometer (V550, produced by JASCO Corporation). The spectra of2PCCzPDBq in the toluene solution of 2PCCzPDBq were measured with thetoluene solution of 2PCCzPDBq put in a quartz cell. The spectra of2PCCzPDBq in the thin film of 2PCCzPDBq were measured with a samplefabricated by deposition of 2PCCzPDBq on a quartz substrate byevaporation. Note that in the case of the absorption spectrum of2PCCzPDBq in the toluene solution of 2PCCzPDBq, the absorption spectrumobtained by subtraction of the absorption spectra of the quartz cell andtoluene from the measured spectrum is shown, and in the case of theabsorption spectrum of 2PCCzPDBq in the thin film of 2PCCzPDBq, theabsorption spectrum obtained by subtraction of the absorption spectrumof the quartz substrate from the measured spectrum is shown.

As shown in FIG. 11, in the case of 2PCCzPDBq in the toluene solution of2PCCzPDBq, the absorption peaks were observed at approximately 305 nmand 385 nm, and the emission wavelength peak was observed at 450 nm(excitation wavelength: 305 nm). As shown in FIG. 12, in the case of2PCCzPDBq in the thin film of 2PCCzPDBq, the absorption peaks wereobserved at approximately 209 nm, 258 nm, 307 nm, 336 nm, and 396 nm,and the emission wavelength peak was observed at 502 nm (excitationwavelength: 396 nm).

Example 2 Synthesis Example 2

In this example, as a synthesis method of one embodiment of the presentinvention, a synthesis method of2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzPDBq, represented by the structural formula (101))will be described. The structure of 2mPCCzPDBq is shown below.

<Synthesis of 2mPCCzPDBq>

First, 1.7 g (5.0 mmol) of 2-(3-chlorophenyl)dibenzo[f,h]quinoxaline,2.0 g (5.0 mmol) of 3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.96 g(10 mmol) of sodium-tert-butoxide, and 41 mg (0.10 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos)were put in a 200-mL three-neck flask and mixed, and the air in theflask was replaced with nitrogen. To this mixture was added 25 mL ofmesitylene, and the resulting mixture was degassed by being stirredwhile the pressure in the flask was reduced.

Then, 29 mg (0.050 mmol) of bis(dibenzylideneacetone)palladium(0)(abbreviation: Pd(dba)₂) was added to the mixture. This mixture wasstirred at 150° C. for 5 hours under a nitrogen stream, so that a solidwas precipitated. The precipitated solid was collected by suctionfiltration. The collected solid was dissolved in approximately 400 mL ofhot toluene, and this solution was suction-filtered through a stack ofCelite and alumina. The resulting filtrate was concentrated to give asolid. The solid was recrystallized with toluene to give 2.8 g of ayellow powder, which was the target substance, in a yield of 79%.

By a train sublimation method, 2.2 g of the obtained yellow powderedsolid, which was the target substance, was purified. The sublimationpurification was carried out at 360° C. under a pressure of 2.5 Pa witha flow rate of an argon gas at 10 mL/min. After the sublimationpurification, 1.2 g of a yellow glassy solid of 2mPCCzPDBq was obtainedat a collection rate of 55%. The synthesis scheme of this step is shownin the following scheme (b-1).

Analysis results by nuclear magnetic resonance spectroscopy (¹H-NMR) ofthe yellow powdered solid obtained in the above step will be describedbelow. ¹H-NMR charts are shown in FIGS. 13A and 13B. FIG. 13B is a chartin which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ)in FIG. 13A is enlarged. These results show that2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzPDBq, represented by the structural formula (101))was obtained in the above step.

¹H-NMR (CDCl₃, 500 MHz): δ(ppm)=7.31-7.54 (m, 7H), 7.58-7.67 (m, 6H),7.73-7.90 (m, 8H), 8.25 (d, J=8.0 Hz, 1H), 8.30 (d, J=8.1 Hz, 1H), 8.45(d, J=6.3 Hz, 1H), 8.49 (d, J=3.7 Hz, 2H), 8.65-8.68 (m, 3H), 9.25 (d,J=2.0 Hz, 1H), 9.37 (d, J=6.9 Hz, 1H), 9.48 (s, 1H).

Then, FIG. 14 shows the absorption spectrum and the emission spectrum of2mPCCzPDBq in a toluene solution of 2mPCCzPDBq, and FIG. 15 shows theabsorption spectrum and the emission spectrum of 2mPCCzPDBq in a thinfilm of 2mPCCzPDBq. The spectra were measured with a UV-visiblespectrophotometer (V550, produced by JASCO Corporation). The spectra of2mPCCzPDBq in the toluene solution of 2mPCCzPDBq were measured with thetoluene solution of 2mPCCzPDBq put in a quartz cell. The spectra of2mPCCzPDBq in the thin film of 2mPCCzPDBq were measured with a samplefabricated by deposition of 2mPCCzPDBq on a quartz substrate byevaporation. Note that in the case of the absorption spectrum of2mPCCzPDBq in the toluene solution of 2mPCCzPDBq, the absorptionspectrum obtained by subtraction of the absorption spectra of the quartzcell and toluene from the measured spectrum is shown, and in the case ofthe absorption spectrum of 2mPCCzPDBq in the thin film of 2mPCCzPDBq,the absorption spectrum obtained by subtraction of the absorptionspectrum of the quartz substrate from the measured spectrum is shown.

As shown in FIG. 14, in the case of 2mPCCzPDBq in the toluene solutionof 2mPCCzPDBq, the absorption peaks were observed at approximately 305nm and 374 nm, and the emission wavelength peak was observed at 480 nm(excitation wavelength: 305 nm). As shown in FIG. 15, in the case of2mPCCzPDBq in the thin film of 2mPCCzPDBq, the absorption peaks wereobserved at approximately 208 nm, 257 nm, 308 nm, 361 nm, and 379 nm,and the emission wavelength peak was observed at 515 nm (excitationwavelength: 380 nm).

Example 3 Synthesis Example 3

In this example, as a synthesis method of one embodiment of the presentinvention, a synthesis method of2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2PCCzPDBq-02, represented by the structural formula(102)) will be described. The structure of 2PCCzPDBq-02 is shown below.

<Synthesis of 2PCCzPDBq-02>

First, 1.4 g (4.2 mmol) of 2-(4-chlorophenyl)dibenzo[f,h]quinoxaline,1.7 g (4.2 mmol) of 2-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.81 g(8.4 mmol) of sodium-tert-butoxide, and 34 mg (0.10 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos)were put in a 200-mL three-neck flask and mixed, and the air in theflask was replaced with nitrogen. To this mixture was added 21 mL ofmesitylene, and the resulting mixture was degassed by being stirredwhile the pressure in the flask was reduced.

Then, 24 mg (0.042 mmol) of bis(dibenzylideneacetone)palladium(0)(abbreviation: Pd(dba)₂) was added to the mixture. This mixture wasstirred at 150° C. for 5 hours under a nitrogen stream, so that a solidwas precipitated. The precipitated solid was collected by suctionfiltration. The collected solid was dissolved in approximately 400 mL ofhot toluene, and this solution was suction-filtered through a stack ofCelite and alumina. The resulting filtrate was concentrated to give asolid. The solid was recrystallized with toluene to give 2.5 g of ayellow powder, which was the target substance, in a yield of 84%.

By a train sublimation method, 2.0 g of the obtained yellow powderedsolid, which was the target substance, was purified. The sublimationpurification was carried out at 390° C. under a pressure of 3.7 Pa witha flow rate of an argon gas at 10 mL/min. After the sublimationpurification, 1.7 g of a yellow glassy solid of 2PCCzPDBq-02 wasobtained at a collection rate of 85%. The synthesis scheme of this stepis shown in the following scheme (c-1).

Analysis results by nuclear magnetic resonance spectroscopy (¹H-NMR) ofthe yellow powdered solid obtained in the above step will be describedbelow. ¹H-NMR charts are shown in FIGS. 16A and 16B. FIG. 16B is a chartin which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ)in FIG. 16A is enlarged. These results show that2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2PCCzPDBq-02, represented by the structural formula(102)) was obtained in the above step.

δ=7.25-7.48 (m, 7H), 7.71-7.75 (m, 2H), 7.71-7.75 (m, 5H), 7.91 (d,J=8.6 Hz, 2H), 8.21 (d, J=7.4 Hz, 1H), 8.26 (d, J=8.0 Hz, 2H), 8.42 (sd,J=1.7 Hz, 2H), 8.63-8.69 (m, 4H), 9.27 (d, J=8.0 Hz, 1H), 9.46 (d, J=6.3Hz, 1H), 9.51 (s, 1H).

Then, FIG. 17 shows the absorption spectrum and the emission spectrum of2PCCzPDBq-02 in a toluene solution of 2PCCzPDBq-02, and FIG. 18 showsthe absorption spectrum and the emission spectrum of 2PCCzPDBq-02 in athin film of 2PCCzPDBq-02. The spectra were measured with a UV-visiblespectrophotometer (V550, produced by JASCO Corporation). The spectra of2PCCzPDBq-02 in the toluene solution of 2PCCzPDBq-02 were measured withthe toluene solution of 2PCCzPDBq-02 put in a quartz cell. The spectraof 2PCCzPDBq-02 in the thin film of 2PCCzPDBq-02 were measured with asample fabricated by deposition of 2PCCzPDBq-02 on a quartz substrate byevaporation. Note that in the case of the absorption spectrum of2PCCzPDBq-02 in the toluene solution of 2PCCzPDBq-02, the absorptionspectrum obtained by subtraction of the absorption spectra of the quartzcell and toluene from the measured spectrum is shown, and in the case ofthe absorption spectrum of 2PCCzPDBq-02 in the thin film of2PCCzPDBq-02, the absorption spectrum obtained by subtraction of theabsorption spectrum of the quartz substrate from the measured spectrumis shown.

As shown in FIG. 17, in the case of 2PCCzPDBq-02 in the toluene solutionof 2PCCzPDBq-02, the absorption peaks were observed at approximately 323nm and 381 nm, and the emission wavelength peak was observed at 421 nm(excitation wavelength: 320 nm). As shown in FIG. 18, in the case of2PCCzPDBq-02 in the thin film of 2PCCzPDBq-02, the absorption peaks wereobserved at approximately 209 nm, 257 nm, 311 nm, 326 nm, 351 nm, and389 nm, and the emission wavelength peak was observed at 473 nm(excitation wavelength: 396 nm).

Example 4 Synthesis Example 4

In this example, as a synthesis method of one embodiment of the presentinvention, a synthesis method of2-{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzPDBq-02, represented by the structuralformula (103)) will be described. The structure of 2mPCCzPDBq-02 isshown below.

<Synthesis of 2mPCCzPDBq-02>

First, 1.7 g (5.0 mmol) of 2-(3-chlorophenyl)dibenzo[f,h]quinoxaline,2.0 g (5.0 mmol) of 2-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, 0.96 g(10 mmol) of sodium-tert-butoxide, and 41 mg (0.10 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos)were put in a 200-mL three-neck flask and mixed, and the air in theflask was replaced with nitrogen. To this mixture was added 25 mL ofmesitylene, and the resulting mixture was degassed by being stirredwhile the pressure in the flask was reduced.

Then, 29 mg (0.050 mmol) of bis(dibenzylideneacetone)palladium(0)(abbreviation: Pd(dba)₂) was added to the mixture. This mixture wasstirred at 150° C. for 4 hours under a nitrogen stream, so that a solidwas precipitated. The precipitated solid was collected by suctionfiltration. The collected solid was dissolved in approximately 400 mL ofhot toluene, and this solution was suction-filtered through a stack ofCelite, alumina, and Florisil. The resulting filtrate was concentratedto give a solid. The solid was recrystallized with toluene to give 3.1 gof a white powder, which was the target substance, in a yield of 87%.

By a train sublimation method, 3.0 g of the obtained white powderedsolid, which was the target substance, was purified. The sublimationpurification was carried out at 360° C. under a pressure of 10 Pa with aflow rate of an argon gas at 5.0 mL/min. After the sublimationpurification, 2.0 g of a yellow glassy solid of 2mPCCzPDBq-02 wasobtained at a collection rate of 65%. The synthesis scheme of this stepis shown in the following scheme (d-1).

Analysis results by nuclear magnetic resonance spectroscopy (¹H-NMR) ofthe white powdered solid obtained in the above step will be describedbelow. ¹H-NMR charts are shown in FIGS. 19A and 19B. FIG. 19B is a chartin which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (6)in FIG. 19A is enlarged. These results show that 2mPCCzPDBq-02(represented by the structural formula (103)) was obtained in the abovestep.

¹H-NMR (DMSO-d⁶, 500 MHz): δ (ppm)=7.17 (t, J1=7.5 Hz, 1H), 7.31-7.39(m, 4H), 7.47-7.52 (m, 2H), 7.56-7.57 (m, 3H), 7.63-7.66 (m, 3H),7.75-7.92 (m, 7H), 7.98 (t, J1=2.5 Hz, 1H), 8.19 (d, J1=7.5 Hz, 1H),8.30 (d, J1=7.5 Hz, 1H), 8.37 (d, J1=8.0 Hz, 1H), 8.54 (sd, J1=1.5 Hz,1H), 8.62 (d, J1=8.0 Hz, 1H), 8.79-8.82 (m, 3H), 9.19 (d, J1=8.0 Hz,1H), 9.25 (d, J1=9.0 Hz, 1H), 9.75 (s, 1H).

Then, FIG. 20 shows the absorption spectrum and the emission spectrum of2mPCCzPDBq-02 in a toluene solution of 2mPCCzPDBq-02, and FIG. 21 showsthe absorption spectrum and the emission spectrum of 2mPCCzPDBq-02 in athin film of 2mPCCzPDBq-02. The spectra were measured with a UV-visiblespectrophotometer (V550, produced by JASCO Corporation). The spectra of2mPCCzPDBq-02 in the toluene solution of 2mPCCzPDBq-02 were measuredwith the toluene solution of 2mPCCzPDBq-02 put in a quartz cell. Thespectra of 2mPCCzPDBq-02 in the thin film of 2mPCCzPDBq-02 were measuredwith a sample fabricated by deposition of 2mPCCzPDBq-02 on a quartzsubstrate by evaporation. Note that in the case of the absorptionspectrum of 2mPCCzPDBq-02 in the toluene solution of 2mPCCzPDBq-02, theabsorption spectrum obtained by subtraction of the absorption spectra ofthe quartz cell and toluene from the measured spectrum is shown, and inthe case of the absorption spectrum of 2mPCCzPDBq-02 in the thin film of2mPCCzPDBq-02, the absorption spectrum obtained by subtraction of theabsorption spectrum of the quartz substrate from the measured spectrumis shown.

As shown in FIG. 20, in the case of 2mPCCzPDBq-02 in the toluenesolution of 2mPCCzPDBq-02, the absorption peaks were observed atapproximately 281 nm, 305 nm, 319 nm, and 374 nm, and the emissionwavelength peaks were observed at 389 nm and 410 nm. As shown in FIG.21, in the case of 2mPCCzPDBq-02 in the thin film of 2mPCCzPDBq-02, theabsorption peaks were observed at approximately 209 nm, 257 nm, 309 nm,327 nm, 354 nm, and 386 nm, and the emission wavelength peak wasobserved at 484 nm (excitation wavelength: 381 nm).

Example 5

In this example, a light-emitting element 1 and a light-emitting element2 each containing a dibenzo[f,h]quinoxaline derivative which is oneembodiment of the present invention and a comparative light-emittingelement 3 were fabricated. The structure of each light-emitting elementwill be described with reference to FIG. 22. Chemical formulae ofmaterials used in this example are shown below.

<<Fabrication of Light-Emitting Element 1, Light-Emitting Element 2, andComparative Light-Emitting Element 3>>

First, indium tin oxide containing silicon oxide (ITSO) was depositedover a glass substrate 1100 by a sputtering method, so that a firstelectrode 1101 functioning as an anode was formed. Note that thethickness of the first electrode 1101 was set to be 110 nm and that thearea of the first electrode 1101 was set to be 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 1, thelight-emitting element 2, and the comparative light-emitting element 3over the substrate 1100, a surface of the substrate was washed withwater, baking was performed at 200° C. for 1 hour, and then UV ozonetreatment was performed for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus in which the pressure had been reduced to approximately 10⁻⁴Pa, and subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate 1100 over whichthe first electrode 1101 was formed faced downward. In this example, acase will be described where a hole-injection layer 1111, ahole-transport layer 1112, a light-emitting layer 1113, anelectron-transport layer 1114, and an electron-injection layer 1115which are included in an EL layer 1102 were sequentially formed by avacuum evaporation method.

After reducing the pressure of the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide were deposited by co-evaporation so that the mass ratioof DBT3P-II to molybdenum oxide was 4:2, whereby the hole-injectionlayer 1111 was formed on the first electrode 1101. The thickness of thehole-injection layer 1111 was set to be 20 nm. Note that co-evaporationis an evaporation method in which a plurality of different substancesare concurrently vaporized from their respective evaporation sources.

Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was deposited by evaporation to a thickness of 20 nm, wherebythe hole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 was formed on the hole-transportlayer 1112. In the case of the light-emitting element 1,2-{3-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzPDBq-02, represented by the structural formula(103)),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) were deposited to a thickness of 20nm by co-evaporation, so that the mass ratio of 2mPCCzPDBq-02 to PCBBiFand [Ir(tBuppm)₂(acac)] was 0.7:0.3:0.05. Then, 2mPCCzPDBq-02, PCBBiF,and [Ir(tBuppm)₂(acac)] were deposited to a thickness of 20 nm byco-evaporation, so that the mass ratio of 2mPCCzPDBq-02 to PCBBiF and[Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05. In this manner, the light-emittinglayer 1113 having a stacked structure and a thickness of 40 nm wasformed.

In the case of the light-emitting element 2,2-{3-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzPDBq, represented by the structural formula (101)),PCBBiF, and [Ir(tBuppm)₂(acac)] were deposited to a thickness of 20 nmby co-evaporation, so that the mass ratio of 2mPCCzPDBq to PCBBiF and[Ir(tBuppm)₂(acac)] was 0.7:0.3:0.05. Then, 2mPCCzPDBq, PCBBiF, and[Ir(tBuppm)₂(acac)] were deposited to a thickness of 20 nm byco-evaporation, so that the mass ratio of 2mPCCzPDBq to PCBBiF and[Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05. In this manner, the light-emittinglayer 1113 having a stacked structure and a thickness of 40 nm wasformed.

In the case of the comparative light-emitting element 3,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II), PCBBiF, and [Ir(tBuppm)₂(acac)] weredeposited to a thickness of 20 nm by co-evaporation, so that the massratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was0.7:0.3:0.05. Then, 2mDBTBPDBq-II, PCBBiF, and [Ir(tBuppm)₂(acac)] weredeposited to a thickness of 20 nm by co-evaporation, so that the massratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was0.8:0.2:0.05. In this manner, the light-emitting layer 1113 having astacked structure and a thickness of 40 nm was formed.

Then, in the case of the light-emitting element 1, 2mPCCzPDBq-02 wasdeposited to a thickness of 20 nm on the light-emitting layer 1113 byevaporation and bathophenanthroline (abbreviation: Bphen) was thendeposited to a thickness of 10 nm by evaporation, whereby theelectron-transport layer 1114 was formed. In the case of thelight-emitting element 2, 2mPCCzPDBq was deposited to a thickness of 20nm on the light-emitting layer 1113 by evaporation andbathophenanthroline (abbreviation: Bphen) was then deposited to athickness of 10 nm by evaporation, whereby the electron-transport layer1114 was formed. In the case of the comparative light-emitting element3, 2mDBTBPDBq-II was deposited to a thickness of 20 nm on thelight-emitting layer 1113 by evaporation and bathophenanthroline(abbreviation: Bphen) was then deposited to a thickness of 10 nm byevaporation, whereby the electron-transport layer 1114 was formed.

Furthermore, lithium fluoride was deposited to a thickness of 1 nm onthe electron-transport layer 1114 by evaporation, whereby theelectron-injection layer 1115 was formed.

Finally, aluminum was deposited to a thickness of 200 nm on theelectron-injection layer 1115 by evaporation to form a second electrode1103 serving as a cathode; thus, the light-emitting element 1, thelight-emitting element 2, and the comparative light-emitting element 3were fabricated. Note that in all the above evaporation steps,evaporation was performed by a resistance-heating method.

Table 1 shows the element structures of the light-emitting element 1,the light-emitting element 2, and the comparative light-emitting element3 that were fabricated as described above.

TABLE 1 Hole- Light- Electron- First Hole-injection transport emittinginjection Second Electrode Layer Layer Layer Electron-transport LayerLayer Electrode Light- ITSO DBT3P-II:MoOx BPAFLP * 2mPCCzPDBq-02 BphenLiF Al emitting (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200nm) Element 1 Light- ITSO DBT3P-II:MoOx BPAFLP ** 2mPCCzPDBq Bphen LiFAl emitting (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200 nm)Element 2 Comparative ITSO DBT3P-II:MoOx BPAFLP *** 2mDBTBPDBq-II BphenLiF Al Light- (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200nm) emitting Element 3 * 2mPCCzPDBq-02:PCBBiF:[Ir(tBuppm)₂(acac)](0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) **2mPCCzPDBq:PCBBiF:[Ir(tBuppm)₂(acac)] (0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05(20 nm)) *** 2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)] (0.7:0.3:0.05 (20nm)\0.8:0.2:0.05 (20 nm))

The fabricated light-emitting element 1, light-emitting element 2, andcomparative light-emitting element 3 were sealed in a glove box under anitrogen atmosphere so as not to be exposed to the air (specifically, asealant was applied to surround the elements, UV treatment wasperformed, and heat treatment was performed at 80° C. for 1 hour).

<<Operation Characteristics of Light-Emitting Element 1, Light-EmittingElement 2, and Comparative Light-Emitting Element 3>>

Operation characteristics of the fabricated light-emitting element 1,light-emitting element 2, and comparative light-emitting element 3 weremeasured. Note that the measurements were performed at room temperature(in an atmosphere kept at 25° C.). Results are shown in FIG. 23, FIG.24, FIG. 25, and FIG. 26.

Table 2 shows initial values of main characteristics of thelight-emitting element 1, the light-emitting element 2, and thecomparative light-emitting element 3 at a luminance of approximately1000 cd/m²

TABLE 2 External Current Current Power Quantum Voltage Current DensityChromaticity Luminance Efficiency Efficiency Efficiency (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 2.9 0.038 1.0(0.42, 0.57) 1100 110 120 29 Element 1 Light-emitting 2.8 0.033 0.8(0.41, 0.58) 920 110 120 29 Element 2 Comparative 2.9 0.035 0.88 (0.42,0.57) 980 110 120 30 Light-emitting Element 3

FIG. 27 shows the emission spectra of the light-emitting element 1, thelight-emitting element 2, and the comparative light-emitting element 3,through which a current flows at a current density of 2.5 mA/cm². Asshown in FIG. 27, the emission spectra of the light-emitting element 1,the light-emitting element 2, and the comparative light-emitting element3 each have a peak at approximately 546 nm, which is attributed to[Ir(tBuppm)₂(acac)].

FIG. 28A shows results of reliability tests on the light-emittingelement 1, the light-emitting element 2, and the comparativelight-emitting element 3. In FIG. 28A, the vertical axis representsnormalized luminance (%) on the assumption that the initial luminance is100%, and the horizontal axis represents driving time (h) of theelements. Note that in the reliability tests, the light-emitting element1, the light-emitting element 2, and the comparative light-emittingelement 3 were driven under the conditions where the initial luminancewas set to be 5000 cd/m² and the current density was constant.

The results show that the light-emitting element 1 fabricated using2mPCCzPDBq-02 and the light-emitting element 2 fabricated using2mPCCzPDBq, which are embodiments of the present invention, have ahigher reliability and a longer lifetime than the comparativelight-emitting element 3 fabricated using 2mDBTBPDBq-II.

FIG. 28B shows measurement results of the amount of change in voltage atthe reliability tests. The vertical axis represents the amount of changein voltage (V), and the horizontal axis represents driving time (h) ofthe elements. These results show that the amount of increase in voltagein each of the light-emitting element 1 and the light-emitting element 2which were driven at constant current is smaller than that in thecomparative light-emitting element 3. For example, after thelight-emitting elements were driven for approximately 500 hours, theamount of increase in voltage in the comparative light-emitting element3 is approximately 0.08 V, whereas those in the light-emitting elements1 and 2 are approximately 0.05 V and 0.02 V, respectively. That is, theamount of increase in voltage in the light-emitting element 1 and thatin the light-emitting element 2 are approximately as small as ½ and ¼,respectively, of that in the comparative light-emitting element 3, whichindicates a significant effect of one embodiment of the presentinvention.

Note that the combination of PCBBiF and each of 2mPCCzPDBq-02,2mPCCzPDBq, and 2mDBTBPDBq-II forms an exciplex (because the mixed filmcontaining PCBBiF and any of these dibenzoquinoxaline compounds exhibitsyellow-green light emission having a longer wavelength than the filmcontaining only PCBBiF or the film containing only any of thesedibenzoquinoxaline compounds). Furthermore, the HOMO levels of2mPCCzPDBq-02, 2mPCCzPDBq, 2mDBTBPDBq-II, and PCBBiF are −5.69 eV, −5.63eV, −6.22 eV, and −5.36 eV, respectively. The HOMO levels were obtainedthrough a cyclic voltammetry (CV) measurement.

By using the HOMO levels obtained as described above, ΔE_(HOMO) in thelight-emitting layer of each light-emitting element was calculated.Table 3 shows the results.

TABLE 3 ΔE_(HOMO) (eV) Light-emitting Element 1 0.33 Light-emittingElement 2 0.27 Comparative Light-emitting Element 3 0.86

According to the results, it is important that ΔE_(HOMO) is less than orequal to 0.4 eV, preferably less than or equal to 0.3 eV.

Furthermore, the HOMO level of BPAFLP used for the hole-transport layeris −5.51 eV. Therefore, it is found that the HOMO level of the thirdorganic compound used for the hole-transport layer is lower than theHOMO level of PCBBiF that is the second organic compound and is locatedbetween the HOMO level of PCBBiF that is the second organic compound andthe HOMO level of the first organic compound (2mPCCzPDBq-02 or2mPCCzPDBq). This is important because holes are injected not only intothe second organic compound but also partly into the first organiccompound.

Example 6

In this example, a light-emitting element 4 containing adibenzo[f,h]quinoxaline derivative which is one embodiment of thepresent invention was fabricated. The structure of the light-emittingelement will be described with reference to FIG. 22 as in Example 5.Chemical formulae of materials used in this example are shown below.

<<Fabrication of Light-Emitting Element 4>>

First, indium tin oxide containing silicon oxide (ITSO) was depositedover a glass substrate 1100 by a sputtering method, so that a firstelectrode 1101 functioning as an anode was formed. Note that thethickness of the first electrode 1101 was set to be 110 nm and that thearea of the first electrode 1101 was set to be 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 4 over thesubstrate 1100, a surface of the substrate was washed with water, bakingwas performed at 200° C. for 1 hour, and then UV ozone treatment wasperformed for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus in which the pressure had been reduced to approximately 10⁻⁴Pa, and subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate 1100 over whichthe first electrode 1101 was formed faced downward. In this example, acase will be described where a hole-injection layer 1111, ahole-transport layer 1112, a light-emitting layer 1113, anelectron-transport layer 1114, and an electron-injection layer 1115which are included in an EL layer 1102 were sequentially formed by avacuum evaporation method.

After reducing the pressure of the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide were deposited by co-evaporation so that the mass ratioof DBT3P-II to molybdenum oxide was 4:2, whereby the hole-injectionlayer 1111 was formed on the first electrode 1101. The thickness of thehole-injection layer 1111 was set to be 20 nm. Note that co-evaporationis an evaporation method in which a plurality of different substancesare concurrently vaporized from their respective evaporation sources.

Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was deposited to a thickness of 20 nm by evaporation, wherebythe hole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 was formed on the hole-transportlayer 1112. By co-evaporation,2-{4-[2-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}dibenzo[f,h]quinoxaline(abbreviation: 2PCCzPDBq-02, represented by the structural formula(102)),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]) were deposited to a thickness of 20nm, so that the mass ratio of 2PCCzPDBq-02 to PCBBiF and[Ir(dppm)₂(acac)] was 0.7:0.3:0.05. Then, 2PCCzPDBq-02, PCBBiF, and[Ir(dppm)₂(acac)] were deposited to a thickness of 20 nm byco-evaporation, so that the mass ratio of 2PCCzPDBq-02 to PCBBiF and[Ir(dppm)₂(acac)] was 0.8:0.2:0.05. In this manner, the light-emittinglayer 1113 having a stacked structure and a thickness of 40 nm wasformed.

Then, 2PCCzPDBq-02 was deposited to a thickness of 20 nm on thelight-emitting layer 1113 by evaporation, and then bathophenanthroline(abbreviation: Bphen) was deposited to a thickness of 10 nm byevaporation, whereby the electron-transport layer 1114 was formed.

Furthermore, lithium fluoride was deposited to a thickness of 1 nm onthe electron-transport layer 1114 by evaporation, whereby theelectron-injection layer 1115 was formed.

Finally, aluminum was deposited to a thickness of 200 nm on theelectron-injection layer 1115 by evaporation to form the secondelectrode 1103 serving as a cathode; thus, the light-emitting element 4was fabricated. Note that in all the above evaporation steps,evaporation was performed by a resistance-heating method.

Table 4 shows the element structure of the light-emitting element 4 thatwas fabricated as described above.

TABLE 4 Hole- Light- Electron- First Hole-injection transport emittinginjection Second Electrode Layer Layer Layer Electron-transport LayerLayer Electrode Light- ITSO DBT3P-II:MoOx BPAFLP * 2PCCzPDBq-02 BphenLiF Al emitting (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200nm) Element 4 * 2PCCzPDBq-02:PCBBiF:[Ir(dppm)₂(acac)] (0.7:0.3:0.05 (20nm)\0.8:0.2:0.05 (20 nm))

The fabricated light-emitting element 4 was sealed in a glove box undera nitrogen atmosphere so as not to be exposed to the air (specifically,a sealant was applied to surround the element, UV treatment wasperformed, and heat treatment was performed at 80° C. for 1 hour).

<<Operation Characteristics of Light-Emitting Element 4>>

Operation characteristics of the fabricated light-emitting element 4were measured. Note that the measurements were performed at roomtemperature (in an atmosphere kept at 25° C.). Results are shown in FIG.29, FIG. 30, FIG. 31, and FIG. 32.

Table 5 shows initial values of main characteristics of thelight-emitting element 4 at a luminance of approximately 1000 cd/m².

TABLE 5 External Current Current Power Quantum Voltage Current DensityChromaticity Luminance Efficiency Efficiency Efficiency (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) Light- 2.8 0.045 1.1 (0.56,0.44) 910 81 91 30 emitting Element 4

FIG. 33 shows the emission spectrum of the light-emitting element 4through which current flows at a current density of 2.5 mA/cm². As shownin FIG. 33, the emission spectrum of the light-emitting element 4 has apeak at approximately 581 nm, which is attributed to [Ir(dppm)₂(acac)].

FIG. 34A shows results of a reliability test on the light-emittingelement 4. In FIG. 34A, the vertical axis represents normalizedluminance (%) on the assumption that the initial luminance is 100%, andthe horizontal axis represents driving time (h) of the element. Notethat in the reliability test, the light-emitting element 4 was drivenunder the conditions where the initial luminance was set to be 5000cd/m² and the current density was constant.

The results show that the light-emitting element 4 fabricated using2PCCzPDBq-02, which is one embodiment of the present invention, has ahigh reliability and a long lifetime.

FIG. 34B shows measurement results of the amount of change in voltage atthe reliability test. The vertical axis represents the amount of changein voltage (V), and the horizontal axis represents driving time (h) ofthe element. The results show that the amount of increase in voltage inthe light-emitting element 4 which was driven at constant current issmall. For example, after the light-emitting element 4 was driven forapproximately 500 hours, the amount of increase in voltage isapproximately 0.01 V. A comparative light-emitting element 9 wasfabricated using 2mDBTBPDBq-II instead of 2PCCzPDBq-02 of thelight-emitting element 4 and driven under conditions similar to those ofthe light-emitting element 4. In that case, the amount of increase involtage was approximately 0.06 V after the comparative light-emittingelement 9 was driven for approximately 500 hours. That is, the amount ofincrease in voltage in the light-emitting element 4 is approximately assmall as ⅙ of that in the comparative light-emitting element 9, whichindicates a significant effect of one embodiment of the presentinvention.

Note that the combination of 2PCCzPDBq-02 and PCBBiF forms an exciplex(because the mixed film containing this dibenzoquinoxaline compound andPCBBiF exhibits green light emission having a longer wavelength than thefilm containing only this dibenzoquinoxaline compound or the filmcontaining only PCBBiF). Furthermore, since the HOMO level of2PCCzPDBq-02 is −5.68 eV, ΔE_(HOMO) in the light-emitting layer of thelight-emitting element 4 is 0.32 eV. Accordingly, it is important thatΔE_(HOMO) is less than or equal to 0.4 eV.

Furthermore, the HOMO level of BPAFLP used for the hole-transport layeris −5.51 eV. Therefore, it is found that the HOMO level of the thirdorganic compound used for the hole-transport layer is lower than theHOMO level of PCBBiF that is the second organic compound and is locatedbetween the HOMO level of PCBBiF that is the second organic compound andthe HOMO level of the first organic compound (2PCCzPDBq-02). This isimportant because holes are injected not only into the second organiccompound but also partly into the first organic compound.

Example 7 Synthesis Example 5

In this example, as a synthesis method of one embodiment of the presentinvention, a synthesis method of2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quinoxaline (abbreviation: 2mPCCzBPDBq, represented by the structuralformula (122)) will be described. The structure of 2mPCCzBPDBq is shownbelow.

Synthesis of2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quinoxaline(Abbreviation: 2mPCCzBPDBq)

First, 2.0 g (4.3 mmol) of2-(3′-bromobiphenyl-3-yl)dibenzo[f,h]quinoxaline, 1.8 g (4.3 mmol) of3-(9-phenyl-9H-carbazol-3-yl)-9H-carbazole, and 0.83 g (8.6 mmol) ofsodium-tert-butoxide were put in a 100-mL three-neck flask and mixed,and the air in the flask was replaced with nitrogen. To this mixture wasadded 22 mL of mesitylene, and the resulting mixture was degassed bybeing stirred while the pressure in the flask was reduced.

Next, 25 mg (0.040 mmol) of bis(dibenzylideneacetone)palladium(0)(abbreviation: Pd(dba)₂) and 35 mg (0.09 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: S-Phos)were added to this mixture. This mixture was stirred at 150° C. for 23hours under a nitrogen stream. After a predetermined time elapsed, waterand toluene were added to this mixture, and an aqueous layer of theobtained filtrate was subjected to extraction with toluene. The obtainedextract solution and an organic layer were combined, washed with anaqueous solution of sodium hydrogen carbonate and saturated brine, anddried with magnesium sulfate. The obtained mixture was gravity-filtered,and the filtrate was concentrated to give an oily substance. The oilysubstance was dissolved in toluene, and this solution wassuction-filtered through a stack of Celite and alumina. The obtainedfiltrate was concentrated to give a brown oily substance. This oilysubstance was purified by high performance liquid chromatography. Columnchromatography was performed using chloroform as a developing solvent(the pressure: 4.5 MPa, the flow rate: 100 mL/min, the holding time: 45minutes, and the injection amount: 0.9 g/30 mL). The obtained fractionwas concentrated and recrystallized with hexane to give 0.66 g of ayellow powder, which was the target substance, in a yield of 18%.

By a train sublimation method, 0.66 g of the obtained yellow powderedsolid, which was the target substance, was purified. The sublimationpurification was carried out at 385° C. under a pressure of 2.6 Pa witha flow rate of an argon gas at 5 mL/min. After the sublimationpurification, 0.5 g of a yellow glassy solid of 2mPCCzBPDBq was obtainedat a collection rate of 83%. The synthesis scheme of this step is shownin the following scheme (e-1).

Analysis results by nuclear magnetic resonance spectroscopy (¹H-NMR) ofthe yellow powdered solid obtained in the above step will be describedbelow. ¹H-NMR charts are shown in FIGS. 35A and 35B. FIG. 35B is a chartin which the range from 7.0 (ppm) to 10 (ppm) on the horizontal axis (δ)in FIG. 35A is enlarged. These results show that2-{3′-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]biphenyl-3-yl}dibenzo[f,h]quinoxaline(abbreviation: 2mPCCzBPDBq, represented by the structural formula (122))was obtained in the above step.

¹H-NMR (CDCl₃, 500 MHz): δ (ppm)=7.30-7.37 (m, 2H), 7.41-7.53 (m, 5H),7.59-7.89 (m, 18H), 8.04 (dd, J=1.7 Hz, 1H), 8.23 (d, J=7.5 Hz, 1H),8.28 (d, 8.0 Hz, 1H), 8.35 (d, J=8.0 Hz, 1H), 8.48 (dd, J=11.4 Hz, J=1.7Hz, 2H), 8.66 (d, J=8.1 Hz, 1H), 8.70 (s, 1H), 9.25 (dd, J=6.3 Hz, J=1.1Hz, 1H), 9.43 (dd, J=7.5 Hz, J=1.7 Hz, 1H), 9.47 (s, 1H).

Example 8

In this example, a light-emitting element 5 containing thedibenzo[f,h]quinoxaline derivative, 2mPCCzPDBq, which is represented bythe structural formula (101) and is one embodiment of the presentinvention, a comparative light-emitting element 6 containing acomparative material, 2mDBTPDBq-II, and a comparative light-emittingelement 7 containing a comparative material,2-[3-(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mCzPDBq), were fabricated. A fabrication method of each light-emittingelement is basically the same as that in Example 5 and thus omitted.Chemical formulae of materials used in this example are shown below.

<<Fabrication of Light-Emitting Element 5, Comparative Light-EmittingElement 6, and Comparative Light-Emitting Element 7>>

Table 6 shows the element structures of the light-emitting element 5,the comparative light-emitting element 6, and the comparativelight-emitting element 7 fabricated in this example.

TABLE 6 Hole- Light- Electron- First Hole-injection transport emittinginjection Second Electrode Layer Layer Layer Electron-transport LayerLayer Electrode Light- ITO DBT3P-II:MoOx BPAFLP * 2mPCCzPDBq Bphen LiFAl emitting (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200 nm)Element 5 Comparative ITO DBT3P-II:MoOx BPAFLP ** 2mDBTPDBq-II Bphen LiFAl Light- (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200 nm)emitting Element 6 Comparative ITO DBT3P-II:MoOx BPAFLP *** 2mCzPDBqBphen LiF Al Light- (110 nm) (4:2 20 nm) (20 nm) (20 nm) (10 nm) (1 nm)(200 nm) emitting Element 7 * 2mPCCzPDBq:PCBBiF:[Ir(tBuppm)₂(acac)](0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm)) **2mDBTPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)] (0.7:0.3:0.05 (20nm)\0.8:0.2:0.05 (20 nm)) *** 2mCzPDBq:PCBBiF:[Ir(tBuppm)₂(acac)](0.7:0.3:0.05 (20 nm)\0.8:0.2:0.05 (20 nm))

The fabricated light-emitting element 5, comparative light-emittingelement 6, and comparative light-emitting element 7 were sealed in aglove box under a nitrogen atmosphere so as not to be exposed to the air(specifically, a sealant was applied to surround the elements, UVtreatment was performed, and heat treatment was performed at 80° C. for1 hour).

<<Operation Characteristics of Light-Emitting Element 5, ComparativeLight-Emitting Element 6, and Comparative Light-Emitting Element 7>>

Operation characteristics of the fabricated light-emitting element 5,comparative light-emitting element 6, and comparative light-emittingelement 7 were measured. Note that the measurements were performed atroom temperature (in an atmosphere kept at 25° C.).

FIG. 36 shows current density-luminance characteristics, FIG. 37 showsvoltage-luminance characteristics, FIG. 38 shows luminance-currentefficiency characteristics, and FIG. 39 shows voltage-currentcharacteristics of each light-emitting element.

Table 7 shows initial values of main characteristics of thelight-emitting element 5, the comparative light-emitting element 6, andthe comparative light-emitting element 7 at a luminance of approximately1000 cd/m².

TABLE 7 External Current Current Power Quantum Voltage Current DensityChromaticity Luminance Efficiency Efficiency Efficiency (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) Light- 2.8 0.047 1.2 (0.42,0.57) 1100 95 110 25 emitting Element 5 Comparative 3.0 0.044 1.1 (0.43,0.56) 990 90 94 24 Light- emitting Element 6 Comparative 2.9 0.060 1.5(0.41, 0.58) 940 63 68 16 Light- emitting Element 7

FIG. 40 shows the emission spectra of the light-emitting element 5, thecomparative light-emitting element 6, and the comparative light-emittingelement 7, through which a current flows at a current density of 2.5mA/cm². As shown in FIG. 40, the emission spectra of the light-emittingelement 5, the comparative light-emitting element 6, and the comparativelight-emitting element 7 each have a peak at approximately 544 nm, whichis attributed to [Ir(tBuppm)₂(acac)].

FIG. 41A shows results of reliability tests on the light-emittingelement 5, the comparative light-emitting element 6, and the comparativelight-emitting element 7. In FIG. 41A, the vertical axis representsnormalized luminance (%) on the assumption that the initial luminance is100%, and the horizontal axis represents driving time (h) of theelements. Note that in the reliability tests, the light-emitting element5, the comparative light-emitting element 6, and the comparativelight-emitting element 7 were driven under the conditions where theinitial luminance was set to be 5000 cd/m² and the current density wasconstant.

The results show that the light-emitting element 5 fabricated using2mPCCzPDBq, which is one embodiment of the present invention, has ahigher reliability and a longer lifetime than the comparativelight-emitting element 6 fabricated using 2mDBTPDBq-II and thecomparative light-emitting element 7 fabricated using 2mCzPDBq.

FIG. 41B shows measurement results of the amount of change in voltage atthe reliability tests. The vertical axis represents the amount of changein voltage (V), and the horizontal axis represents the driving time (h)of the elements. These results show that the amount of increase involtage in the light-emitting element 5 which was driven at constantcurrent is smaller than those in the comparative light-emitting elements6 and 7. For example, after the light-emitting elements were driven forapproximately 1000 hours, the amount of increase in voltage in thecomparative light-emitting element 6 is approximately 0.31 V and that inthe comparative light-emitting element 7 is approximately 0.50 V,whereas that in the light-emitting element 5 is approximately 0.04 V.That is, the amount of increase in voltage in the light-emitting element5 is much smaller than those in the comparative light-emitting elements6 and 7, which indicates a significant effect of one embodiment of thepresent invention.

Note that the combination of PCBBiF and each of 2mPCCzPDBq,2mDBTPDBq-II, and 2mCzPDBq forms an exciplex (because the mixed filmcontaining PCBBiF and any of these dibenzoquinoxaline compounds exhibitsyellow-green light emission having a longer wavelength than the filmcontaining only PCBBiF or the film containing only any of thesedibenzoquinoxaline compounds). Furthermore, the HOMO levels of2mPCCzPDBq, 2mDBTPDBq-II, 2mCzPDBq, and PCBBiF are −5.63 eV, −6.22 eV,−5.91 eV, and −5.36 eV, respectively. The HOMO levels were obtainedthrough a cyclic voltammetry (CV) measurement.

By using the HOMO levels obtained as described above, ΔE_(HOMO) in thelight-emitting layer of each light-emitting element was calculated.Table 8 shows the results.

TABLE 8 ΔE_(HOMO) (eV) Light-emitting Element 5 0.27 ComparativeLight-emitting Element 6 0.86 Comparative Light-emitting Element 7 0.55

According to the results, it is important that ΔE_(HOMO) is less than orequal to 0.4 eV, preferably less than or equal to 0.3 eV.

Furthermore, the HOMO level of BPAFLP used for the hole-transport layeris −5.51 eV. Therefore, it is found that the HOMO level of the thirdorganic compound used for the hole-transport layer is lower than theHOMO level of PCBBiF that is the second organic compound and is locatedbetween the HOMO level of PCBBiF that is the second organic compound andthe HOMO level of the first organic compound (2mPCCzPDBq). This isimportant because holes are injected not only into the second organiccompound but also partly into the first organic compound.

Example 9

In this example, a light-emitting element 8 containing thedibenzo[f,h]quinoxaline derivative, 2mPCCzBPDBq, which is one embodimentof the present invention, was fabricated. A fabrication method of thelight-emitting element 8 is basically the same as that in Example 5 andthus omitted. Chemical formulae of materials used in this example areshown below.

<<Fabrication of Light-Emitting Element 8>>

Table 9 shows the element structure of the light-emitting element 8fabricated in this example.

TABLE 9 Hole- Light- Electron- First Hole-injection transport emittinginjection Second Electrode Layer Layer Layer Elecron-transport LayerLayer Electrode Light- ITO DBT3P-II:MoOx BPAFLP * 2mPCCzBPDBq Bphen LiFAl emitting (110 nm) (4:2 60 nm) (20 nm) (20 nm) (10 nm) (1 nm) (200 nm)Element 8 * 2mPCCzBPDBq:PCBBiF:[Ir(dppm)₂(acac)] (0.7:0.3:0.05 (20nm)\0.8:0.2:0.05 (20 nm))

The fabricated light-emitting element 8 was sealed in a glove box undera nitrogen atmosphere so as not to be exposed to the air (specifically,a sealant was applied to surround the element, UV treatment wasperformed, and heat treatment was performed at 80° C. for 1 hour).

<<Operation Characteristics of Light-Emitting Element 8>>

Operation characteristics of the fabricated light-emitting element 8were measured. Note that the measurements were performed at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 42 shows current density-luminance characteristics, FIG. 43 showsvoltage-luminance characteristics, FIG. 44 shows luminance-currentefficiency characteristics, and FIG. 45 shows voltage-currentcharacteristics of the light-emitting element 8.

Table 10 shows initial values of main characteristics of thelight-emitting element 8 at a luminance of approximately 1000 cd/m².

TABLE 10 External Current Current Power Quantum Voltage Current DensityChromaticity Luminance Efficiency Efficiency Efficiency (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) Light- 3.0 0.046 1.2 (0.56,0.44) 900 78 82 30 emitting Element 8

FIG. 46 shows the emission spectrum of the light-emitting element 8,through which a current flows at a current density of 2.5 mA/cm². Asshown in FIG. 46, the emission spectrum of the light-emitting element 8has a peak at approximately 584 nm, which is attributed to[Ir(dppm)₂(acac)].

Example 10

In this example, a light-emitting element which is one embodiment of thepresent invention was fabricated and subjected to a preservation test.

In this example, a light-emitting element 1A, a light-emitting element2A, a comparative light-emitting element 3A, a light-emitting element4A, and a light-emitting element 8A were fabricated. The structure andfabrication method of the light-emitting element 1A are the same asthose of the light-emitting element 1 in Example 5. The structures andfabrication methods of the light-emitting element 2A, the comparativelight-emitting element 3A, the light-emitting element 4A, and thelight-emitting element 8A are the same as those of the light-emittingelement 2 in Example 5, the comparative light-emitting element 3 inExample 5, the light-emitting element 4 in Example 6, and thelight-emitting element 8 in Example 9, respectively.

In preservation tests of this example, the light-emitting elements wereeach preserved in a thermostatic oven maintained at 100° C. for apredetermined time, and the operation characteristics were measured.Note that the operation characteristics were measured at roomtemperature (in an atmosphere kept at 25° C.) after the light-emittingelements were taken out of the thermostatic oven.

FIG. 47 shows voltage-current characteristics and FIG. 48 showsluminance-external quantum efficiency characteristics of thelight-emitting element 1A after preservation at 100° C. for apredetermined time. In FIG. 47, the horizontal axis represents voltage(V), and the vertical axis represents current (mA). In FIG. 48, thehorizontal axis represents luminance (cd/m²), and the vertical axisrepresents external quantum efficiency (%).

FIG. 49 shows voltage-current characteristics and FIG. 50 showsluminance-external quantum efficiency characteristics of thelight-emitting element 2A after preservation at 100° C. for apredetermined time. In FIG. 49, the horizontal axis represents voltage(V), and the vertical axis represents current (mA). In FIG. 50, thehorizontal axis represents luminance (cd/m²), and the vertical axisrepresents external quantum efficiency (%).

FIG. 51 shows voltage-current characteristics and FIG. 52 showsluminance-external quantum efficiency characteristics of the comparativelight-emitting element 3A after preservation at 100° C. for apredetermined time. In FIG. 51, the horizontal axis represents voltage(V), and the vertical axis represents current (mA). In FIG. 52, thehorizontal axis represents luminance (cd/m²), and the vertical axisrepresents external quantum efficiency (%). Since light emission couldnot be observed in the light-emitting element preserved over 20 hours,FIG. 52 does not show data on luminance-external quantum efficiencycharacteristics after preservation over 20 hours.

FIG. 53 shows voltage-current characteristics and FIG. 54 showsluminance-external quantum efficiency characteristics of thelight-emitting element 4A after preservation at 100° C. for apredetermined time. In FIG. 53, the horizontal axis represents voltage(V), and the vertical axis represents current (mA). In FIG. 54, thehorizontal axis represents luminance (cd/m²), and the vertical axisrepresents external quantum efficiency (%).

FIG. 55 shows voltage-current characteristics and FIG. 56 showsluminance-external quantum efficiency characteristics of thelight-emitting element 8A after preservation at 100° C. for apredetermined time. In FIG. 55, the horizontal axis represents voltage(V), and the vertical axis represents current (mA). In FIG. 56, thehorizontal axis represents luminance (cd/m²), and the vertical axisrepresents external quantum efficiency (%).

FIGS. 47 to 50 and FIGS. 53 to 56 show that the light-emitting element1A, the light-emitting element 2A, the light-emitting element 4A, andthe light-emitting element 8A suffered only a small change involtage-current characteristics and luminance-external quantumefficiency characteristics even after being preserved at 100° C. for 500hours and that the element characteristics hardly deteriorated throughpreservation at high temperature. In contrast, FIG. 51 and FIG. 52 showthat the comparative light-emitting element 3A suffered a considerablechange in voltage-current characteristics and luminance-external quantumefficiency characteristics after being preserved at 100° C. and that theelement characteristics deteriorated through preservation at hightemperature. As shown in FIG. 51, in the comparative light-emittingelement 3A, an initial insulating property is not maintained after 100hours and leakage current is generated, and FIG. 52 indicates a defect,that is, no light emission of the element. The above results show thatheat resistance of a light-emitting element in the case of preservationat high temperature is significantly increased by using a compound whichis one embodiment of the present invention.

This application is based on Japanese Patent Application serial no.2014-151493 filed with Japan Patent Office on Jul. 25, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: an EL layerbetween an anode and a cathode, wherein the EL layer includes alight-emitting layer, wherein the light-emitting layer contains a firstorganic compound, a second organic compound, and a light-emittingsubstance, wherein a combination of the first organic compound and thesecond organic compound forms an exciplex, wherein the first organiccompound includes a 6-membered nitrogen-containing heteroaromatic ringand a bicarbazole skeleton, wherein a HOMO level of the first organiccompound is lower than a HOMO level of the second organic compound, andwherein a difference between the HOMO level of the first organiccompound and the HOMO level of the second organic compound is less thanor equal to 0.4 eV.
 2. The light-emitting element according to claim 1,wherein the bicarbazole skeleton is a 3,3′-bicarbazole skeleton or a2,3′-bicarbazole skeleton.
 3. The light-emitting element according toclaim 1, wherein the light-emitting substance is a phosphorescentcompound.
 4. The light-emitting element according to claim 1, whereinthe EL layer includes a hole-transport layer, the hole-transport layerbeing in contact with the light-emitting layer and containing a thirdorganic compound, and wherein a HOMO level of the third organic compoundis lower than the HOMO level of the second organic compound.
 5. Thelight-emitting element according to claim 1, wherein the first organiccompound is represented by a general formula (G0),

wherein A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond.
 6. A light-emitting device comprising: thelight-emitting element according to claim 1; and a housing.
 7. Anelectronic device comprising: the light-emitting device according toclaim 6; and a connection terminal or an operation key.
 8. A lightingdevice comprising: the light-emitting element according to claim 1; anda housing.
 9. A light-emitting element comprising: an EL layer betweenan anode and a cathode, wherein the EL layer includes a light-emittinglayer, wherein the light-emitting layer contains a first organiccompound, a second organic compound, and a light-emitting substance,wherein a combination of the first organic compound and the secondorganic compound foal's an exciplex, wherein the first organic compoundincludes a 6-membered nitrogen-containing heteroaromatic ring and abicarbazole skeleton and does not contain a triarylamine skeleton,wherein the second organic compound includes a triarylamine skeleton,and wherein a difference between a HOMO level of the first organiccompound and a HOMO level of the second organic compound is less than orequal to 0.4 eV.
 10. The light-emitting element according to claim 9,wherein the bicarbazole skeleton is a 3,3′-bicarbazole skeleton or a2,3′-bicarbazole skeleton.
 11. The light-emitting element according toclaim 9, wherein the light-emitting substance is a phosphorescentcompound.
 12. The light-emitting element according to claim 9, whereinthe EL layer includes a hole-transport layer, the hole-transport layerbeing in contact with the light-emitting layer and containing a thirdorganic compound, and wherein a HOMO level of the third organic compoundis lower than the HOMO level of the second organic compound.
 13. Thelight-emitting element according to claim 9, wherein the first organiccompound is represented by a general formula (G0),

wherein A represents a dibenzo[f,h]quinoxalinyl group; R¹ to R¹⁵represent independently hydrogen, a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 5 to 7 carbon atoms, or a substituted orunsubstituted aryl group having 6 to 13 carbon atoms; and Ar representsa substituted or unsubstituted arylene group having 6 to 25 carbon atomsor a single bond.
 14. A light-emitting device comprising: thelight-emitting element according to claim 9; and a housing.
 15. Anelectronic device comprising: the light-emitting device according toclaim 14; and a connection terminal or an operation key.
 16. A lightingdevice comprising: the light-emitting element according to claim 9; anda housing.